Small alkenes and conjugated dienes are essential building blocks in the petrochemical industry. They can be oligomerized into larger hydrocarbon chains to produce energy-dense fuels in the range C8 -C16 suitable as jet fuels. 1 Today, small conjugated dienes are mainly produced as by-products in the production of ethylene through steam-cracking of fossil-based naphtha. Thus, great attention has recently been directed to more sustainable routes to small bio-based conjugated dienes through direct production from living organisms (e.g., microbes, algae, and plants) as well as from biomass-derived sugars and plant oils. The direct biotechnological production of C8 -C16 hydrocarbons by microbes is possible, however, this process is limited due to the toxicity of these hydrocarbons as they accumulate in the cells and interfere with the cell membrane integrity. Besides this, the extraction and purification processes of large molecules require energy-intensive procedures leading to increased production costs. As an alternative method to overcome these complicating issues we recently proposed a two-step photobiological-photochemical approach where photosynthetic cyanobacteria produce volatile isoprene directly from CO2, water, and (solar) light followed by its photochemical dimerization into C10 monoterpenoids. The second step is a triplet-sensitized photodimerization that also can be run by solar light. Indeed, the triplet state photosensitized dimerization of conjugated dienes is an established approach within organic photochemistry to produce [2+2], [4+2] and [4+4] cycloadducts, and after hydrogenation, the cycloadducts are attractive as kerosene-type fuels with high energy density. A lifecycle assessment of our photobiological-photochemical approach to bio-jet fuels via isoprene indicated that the climate change impact was about 20% that of fossil-based jet fuels. Now, can this method be applied also for, e.g., 1,3-butadiene?