10,11-Dihydro-11-oxodibenz[b,f]oxepinacetic acids and related compounds were synthesized as potential antiinflammatory agents. Among them, 2-(8-methyl-10,11-dihydro-11-oxodibenz[b,f]oxepin-2-yl)propionic acid (16b) and its thiepin analogue (16c) showed excellent antipyretic activity together with potent antiinflammatory and analgesic properties in biological tests. Structure-activity relationships are discussed.