The reactive ion etching of diamond in O-2, CF4/O-2, CHF3/O-2, O-2/Ar discharges has been examined as a function of bias voltage, flow rate and composition of the gas mixtures. Etching in O-2 and O-2/Ar plasmas (with flow ratio of O-2/Ar greater than or equal to 25%) was characterised by a high etch rate (similar to 35 nm/min) and an increase in surface roughness with rising bias voltage. The CF4/O-2 plasmas also produced a high etch rate (similar to 50 nm/min) but with only minor dependence of roughness on bias voltage. In comparison, the O-2/Ar (with O-2/Ar flow ratio < 25%) and CHF3/O-2 plasmas resulted in a low etch rate (7-10 nm/min). The high and low rate regimes were identified as ionenhanced chemical etching and physical sputtering respectively. Etching in the O-2/Ar plasmas has been attributed to a combination of the two processes dependent on the O-2 content.