Chalcones (,-unsaturated ketones) are effective antitumour agents. It has been proved that having halogen or methoxy groups substituted in various positions of the phenyl ring enhances the activity of chalcones many times. The title compounds, C 21 H 20 O 5 and C 19 H 15 BrO 3 , respectively, were chosen for crystallographic study in order to determine their structures and conformations. In both compounds, the keto group is in the s-cis conformation and is almost planar. There are weak intramolecular interactions in both structures. Comment Chalcones possess multi-protecting, biochemical, antifungal, antimalarial and antifertility activities. The carbonyl group is a characteristic functional group in chalcones. In recent years, the synthesis of polymers having a photosensitive functional group has been an active ®eld of research in polymer science. Monomers having structures similar to the title compounds, 4-[3-(3,4-dimethoxyphenyl)prop-2-enoyl]phenyl methacrylate, (I), and 4-[3-(2-bromophenyl)prop-2-enoyl]phenyl methacrylate, (II), have been polymerized in solution using freeradical initiators (Balaji et al., 2000; Subramanian et al., 2001). These polymers, containing an ,-unsaturated carbonyl group, undergo crosslinking upon irradiation with UV light or an electron beam and are being used as photoresistors (Hyder Ali & Srinivasan, 1997; Rehab & Salahuddin, 1999). These photosensitive polymers ®nd application in the ®elds of integrated circuit technology, printing technology and photocurable coatings (Nagamatzu & Inui, 1977). The keto group is in the s-cis conformation in compounds (I) and (II), as can be seen from the C7ÐC8ÐC9ÐO1 torsion-angle values of 4.7 (3) for (I) and 7.2 (6) for (II). The two phenyl rings make dihedral angles with the keto group of 8.5 (2) and 11.8 (1) for (I), and 4.8 (2) and 7.6 (2) for (II). In general, the bond lengths in these conjugated systems are longer than the double bonds and shorter than the single bonds. In both structures, the bond lengths agree with reported values (Carpy et al., 1978). From the torsion angle values C5ÐC4ÐC7ÐC8 = À176.0 (2) , C4ÐC7ÐC8Ð C9 = 177.5 (2) , C7ÐC8ÐC9ÐC10 = À173.2 (2) and C8Ð C9ÐC10ÐC15 = À167.2 (2) in (I), and C1ÐC6ÐC7ÐC8 = 170.6 (4) , C6ÐC7ÐC8ÐC9 = 178.9 (3) , C7ÐC8ÐC9Ð C10 = À173.8 (4) and C8ÐC9ÐC10ÐC11 = 8.1 (6) in (II), it is clear that the unsaturated ketone system is not strictly planar. This deviation from planarity affects the %-electron conjugation. The H atoms at C7 and C8 are trans. The opening of the C4ÐC7ÐC8 angle to 129.5 (2) in (I) and the C6Ð