Thermo-optical absorbance (TOA) detection using a KrF excimer waveguide laser for detection of benzoic acid, dehydroacetic acid and sorbic acid separated by capillary electrophoresis (CE) was studied. Detection limits were, on average, ten times better than those for on-column UV absorbance methods with CE, and two or more times better than those for UV absorbance with HPLC. The influence of increased laser power on TOA detection sensitivity was found to be strong for benzoic and dehydroacetic acids but quite weak for sorbic acid. It was discovered that photoisomerization of sorbic acid (2,4-hexadienoic acid) occurred readily in the detection volume at moderate laser powers (P(ave) = 3 mW) and increased with slow electroosmotic flows (< 6 cm/min). The TOA method described here shows improved detection sensitivity for CE analyses of compounds having only weak absorptivities (< 5% of maximum) at lambda = 248 nm, and thus demonstrates its utility for determination of a variety of analytes in a single separation.