Microscopic work with single-slot grids requires high-quality support films to span the relatively large gap. The imminent unavailability of the polyvinyl formal Pioloform FN 65, which to date has been used as the standard polyvinyl formal for the generation of support films in transmission electron microscopy (TEM), has necessitated the finding of a substitute material to produce such films. Therefore, we compared the polyvinyl butyral Pioloform BM 18 with the polyvinyl formal Pioloform FN 65 for the production of TEM support films, using operational criteria for assessment. Pioloform BM 18 with the solvent chloroform resulted in support films of unacceptable quality compared with Pioloform FN 65. Adding the softener dibutyl phthalate to the chloroform solvent for Pioloform BM 18 markedly improved the film quality, resulting in support films with high transparency and flexibility, and even greater stability in the electron beam when compared with films of Pioloform FN 65. Pioloform FN 65 also had the disadvantage of requiring highly toxic 1,2-dichloroethane as a solvent, whereas Pioloform BM 18 can be used with chloroform.