Radiation chemical reactions of ^•OH, O^•−, N₃•and e^-_aq with o-and m-hydroxycinnamic acids were studied. The second -order rate constants for the reaction of ^•OH with ortho and meta isomers in buffer solution at pH 7 are 3⋅9±0⋅2×10⁹ and 4⋅4±0⋅3×10⁹dm³mol^–1s^–1respectively. At pH 3 the rate with the ortho isomer was halved (1⋅6±0⋅4×10⁹dm³mol^–1s^–1) but it was unaffected in the case of meta isomer (k=4⋅2±0⋅6×10⁹dm³mol^–1s^–1). The rate constant in the reaction of N₃^• with the ortho isomer is lower by an order of magnitude (k=4⋅9±0⋅4×10⁸dm³mol^–1s^–1). The rates of the reaction of e^–_aq with ortho and meta isomers were found to be diffusion controlled. The transient absorption spectrum measured in the ^•OH with o-hydroxycinnamic acid exhibited an absorption maximum at 360 nm and in meta isomer the spectrum was blue -shifted (330 nm) with a shoulder at 390 nm. A peak at 420 nm was observed in the reaction of O^•−with the o-isomer whereas the meta isomer has a maximum at 390 and a broad shoulder at 450 nm. In the reaction of e ⁻_aq the absorption peaks were centred at 370–380nm in both the isomers. The underlying reaction mechanism is discussed.