GaAs (100) surfaces were etched in citric acid-hydrogen peroxide (C6H8O7:H2O2) solutions with volume ratios ranging from 1:1 to 10:1. Ex situ variable angle spectroscopic ellipsometry measurements (1.5-5.5 eV) were made before and after etching for periods of up to 8 min. Analysis of these data indicated that low volume ratios (less than or equal to 2:1), with corresponding low etch rates, produced a thin interfacial layer (which might represent microscopic roughness) beneath a porous oxide. For a 1:1 ratio the interfacial layer thickness increased with etch time, while for a 2:1 ratio it stabilized at a small value (congruent to 2 nm). Higher volume ratios (greater than or equal to 3:1), associated with much higher etch rates, produced qualitatively different overlayers which were more absorbing and 4-6 nm thick. Their optical constants could be modeled as a mixture of a small fraction (typically 5%-10%) of crystalline arsenic in a low-dielectric medium, using an effective medium approximation. Similar effects were seen in etched Al0.3Ga0.7As surfaces. Low etch rates (volume ratio of 5:1 or less) resulted in nearly normal oxide overlayers. A high etch rate (volume ratio 10:1) resulted in an absorbing layer similar to the one seen on rapidly etched GaAs surfaces, 8-12 nm thick.