Based on the detailed structural and compositional characterization of commercial carrageenans , 7 samples were selected for the production of chemically defined carrageenans. These samples were selected because they show a high structural diversity and are thus well-suited for an evaluation of structure-function relationships. The selected carrageenans included a pure κ-and ι-carrageenan (KC1 and IC1) and five hybrid carrageenans with different structural units (IC3, LC1, LC2, C1, and C2). NMR spectroscopy and partial enzymatic hydrolysis suggested that IC3 and C2 are ι/κ and κ/ι hybrid carrageenans which are mainly composed of consecutive blocks of ι-and κ-units. In contrast, the κ-/ι-carrageenan LC2 contains long blocks of κ-units and only single ι-units or very small sections thereof. The carrageenans LC1 and C1 were hybrid carrageenans with long blocks of κ-units which are intercepted by small blocks of ι-and ν-units. In our previous study, we also demonstrated that the commercial carrageenans show significant variation of the associated cations (potassium, calcium, and sodium) . As described above, this may significantly influence IRI activity, thus, monocationic carrageenans were produced. The focus was on potassium and calcium because these cations are most common in carrageenans due to the manufacturing process. However, the pure κ-and ι-carrageenan samples KC1 and IC1 as well as two hybrid carrageenans (LC1 and LC2) were also converted to their sodium form and analyzed for their IRI activity. In addition, LC1 was also subjected to alkaline treatment to convert ν-units to ιunits (sodium and potassium form). By comparing the IRI activity before and after conversion, the influence of ν-units can be evaluated. The structural composition of all carrageenans was fully characterized to exclude significant changes which could affect the IRI activity.