A series of 1:1 molecular adducts of triphenylphosphine oxide ( tppo ) with substituted phenoxyacetic acids has been prepared and characterized by using infrared, solid-state 1H and 31P n.m.r. spectroscopy, and X-ray powder diffraction methods. The crystal structure of one of these, with the herbicidally active (4-chloro-2-methylphenoxy)acetic acid ( mcpa ),[( tppo )( mcpa )], has been determined by X-ray diffraction methods. This compound is isomorphous with the tppo adduct of (2,4-dichlorophenoxy)acetic acid. The title compound has two independent and different molecular adducts in the asymmetric unit each with strong hydrogen-bonding interactions between the carboxylic acid proton and the phosphoryl oxygen of tppo [O...O, 2.579, 2.647(9)?].