The partial three-dimensional analyses of two crystal modifications of trans-β-2-thienylacrylic acid are reported. The C–C bond lengths have been determined with an e.s.d. of about 0·02 Å; the differences in bond lengths and angles measured independently in the two packing arrangements are not significantly greater than the standard deviations of their measurements.

According to their cell dimensions as well as the geometry of contact of exocyclic [graphic omitted] groups the two modifications belong, respectively, to the β- and γ-type of the cinnamic acid series. The photochemistry of the two forms is discussed in terms of their packing arrangements.