Form I of 4-formylbenzoic acid is stable at low temperature and transforms into form II at about 140 °C. The heat of transition (0.9 kcal mole-1) corresponds to the entropy change of Rln4. The crystals of form I are monoclinic, space group P21/a, Z= 4 with a = 7.28 (3), b = 29.21 (3), c = 3-74 (1)/~,, fl= 121-5 (2) °, and those of form II are triclinic, space group P1, Z= 1 with a = 6"43 (1), b = 7.30 (1), c = 3.81 (1) ~, 0c= 97.4 (2), fl= 91.3 (2), ~= 83.2 (2) °. The structures were solved by the Patterson method, and refined by the block-diagonal least-squares method using visually estimated Cu K~ data. The final R values are 0.100 for 369 and 0.079 for 389 non-zero reflexions for forms I and II respectively. The structure of form I consists of a dimer linked by hydrogen bonds [2-57 (1)/~] between the carboxylic groups. The dimers are held together by C-H... O interactions to form a sheet on the (001) plane. The structure of form II contains fourfold disorder in the arrangement of the carboxylic and aldehydic groups, which results in an apparent centre of symmetry. Infinite chains of molecules linked by hydrogen bonds [2.608 (6)/~,] along the [111] axis are held together side by side to form a sheet parallel to the (101) plane. This structure is similar to that of form I of terephthalic acid.