Zeolites are crystalline, microporous materials renowned for their molecular sieving and catalytic applications. The aluminosilicate framework is assembled from silica and alumina tetrahedra linked through apical oxygen atoms. Charge-compensating cations reside in the zeolite pores to render an overall neutral framework. Existing zeolite materials are often modified to optimise their performance in many applications. For example, cation exchange can be used to tailor the pore architecture of a zeolite to significantly improve its selectivity, adsorption and diffusion properties. We have previously shown that changing the nature of the extra-framework cations in zeolites A (LTA) and Y (FAU) can cause structural modifications, such as altering the size of the unit-cell and pore windows and changing the local environment of aluminium atoms in the framework. In this work, we explore the effect of cation exchange on the pore dimensions, cation locations and water distribution within the 12-ring (12R) channel systems of LTL-type zeolites. Zeolites with the LTL topology have attracted particular attention because of their unique, quasi-1D, 12R channel systems, with free access diameters of 7 Å. The geometric constraints imparted by the LTL framework, and the electrostatic fields generated by non-framework cations, create an ideal host environment for a variety of polar guest materials. For example, donor and acceptor chromophores are readily ordered into supramolecular arrays within the 12R channels, allowing efficient resonance energy transfer to occur. These host-guest materials are of particular interest as artificial antenna systems for light harvesting, with applications in photochemical and optoelectronic devices. The cylindrical shape of the 1D channels not only suppresses the aggregation of adsorbed dye molecules, but can further enhance their photochemical properties. An example can be given for encapsulated proflavine dye molecules which align within the 1D channels and are forced to adopt a bent configuration due to the confinement effects of the framework. This alters the fluorescent properties of the material in comparison to the isolated dye. The importance of non-framework cations in generating composite materials that are stable and water-resistant has also been shown. The stability of fluoroenone-KL materials has been attributed to strong interactions between nonframework potassium cations in the 12R channels and the carbonyl oxygen atoms of the organic dye molecules. The applications currently being investigated for LTL composite materials, ranging from catalysis and optoelectronics, to drug delivery, all make use of the available void space and host-guest interactions within the channel systems. Tuning the zeolite pore dimensions and electrostatic fields via cation-exchange is one way to alter the physical and chemical properties of the material. It has previously been shown that the catalytic activity of platinum-L zeolites can be improved by increasing the size of non-framework cations. Selectivity for the dehydrocyclisation of n-hexane and hydrogenation of benzene increases from Li + to Cs + exchanged Pt/L zeolites. On the other hand, exchange with smaller Mg 2+ cations leads to a greater ethanol adsorption capacity. The small ionic radius and high charge density of non-framework Mg 2+ cations in the pores allows them to bind more strongly to ethanol molecules which restricts their motion. Here, we explore the effect of cation exchange on the pore geometry of zeolite L. Non-framework K + cations are exchanged with larger Cs + and smaller Li + ions to physically alter pore apertures, cation locations and water distribution within the 12R channels. Advanced structural characterisation provides insight into the locations of these extra-framework cations and water molecules. This is fundamental in understanding and exploiting the characteristics of these materials. This work is a step towards tuning LTL-zeolites for better use as functional materials. The 12R, 1D channel of the LTL framework is orientated along the crystallographic c-axis, enclosed by vertical stacks of alternating cancrinite cages (can) and double-6 ring units (d6rs), with smaller, elliptical 8R channels alternating between these columns, Figure . K + cations have previously been shown to reside at four distinct cation sites, labelled A-D in Figure . Site A is located at the centre of the d6r units (z = 0), site B at the centre of the can cages (z = 1/2), site C in the window between the elliptical 8R channels and walls of the 12R channel (z = 1/2) and site D at the periphery of the 12R channel (z = 0).