A mechanistic model for the plasma etching of GaAs is compared with experimentally determined kinetic reaction rates and thermodynamic data. The measurements were made on etching characteristics of radio frequency (rf) and electron cyclotron resonance (ECR) plasmas in alkane (CH4, C2H6, and C3H8 ), hydrogen and noble gas mixtures. The model examines the reaction mechanisms on the substrate surface in terms of sequential adsorption and fragmentation of the precursor alkane molecules and the subsequent desorption of volatile reaction products. Experimental results show GaAs etch rates in rf plasmas to be proportional to the initial alkane concentration, and the order of reaction increases with the number of constituent CHx groups within the alkane precursor molecule. By using different noble gas admixtures it has been possible to determine their effect on etch rates with respect to pure hydrogen admixtures, where results show that etch rates scale with the substitution of He≪H2≪Ar admixtures in CH4 plasmas. GaAs etch rates in ECR/magnetic mirror coupled plasmas show that GaAs etch rates are lower than in rf plasmas, with evidence of enhanced positive photoresist degradation at high microwave powers, and H2 partial pressures.