A dynamic mass spectrometer system is described which utilises fast detection techniques to follow pressure surges of gaseous product initiated by incidence of 30 µs flashes of u.v. photons onto ZnO or TiO2 surfaces pretreated in vacuo at 10–6 Nm–2. A difficulty inherent to the system is appearance of “system” transients corresponding to flash-initiated growth in ion currents at m/e= 44, 28, 16 and 12, corresponding to photo-desorption of carbon oxides from walls of the system. Addition of ZnO samples to the system caused appearance of a new transient at m/e= 32, consistent with photoassisted release of O2 from the ZnO surface. Rutile samples similarly treated did not give rise to a comparable O2 transient. Presence of electron-attaching methyl halides at pressures 10–5-10–3 N m–2 over the ZnO or TiO2 surfaces during flashes resulted in only a small transient at m/e= 19, corresponding to growth of CD3H from CD3I/ZnO with ϕ(CD3H)≈ 10–6. Presence of adsorbed, hole-trapping methanol and ethanol resulted in photo-assisted formation of formaldehyde or acetaldehyde and observed yields correspond approximately to quantum efficiencies of 10–4 for these photoassisted dehydrogenation processes. Use of the deuterated ethanol showed that photodesorption occurred accompanied by H/D exchange involving one deuterium atom in the originally adsorbed alcohol. For flash-illuminated ethanol/ZnO systems the mass distribution of transients could best be accounted for as a combination of flash desorption of alcohol, photoassisted formation of acetaldehyde and photoassisted formation of ethylene. The importance of photodegradation of strongly adsorbed ethanol in these photoassisted surface processes was demonstrated by observations that a ratio of 1 : 4 : 9 observed between them when ethanol was present over the ZnO surface during the flash changed to 1 : 10 : 80 when flashes were deliverd to the surface after all gaseous alcohol was removed. For ethanol/TiO2 systems, flash desorption of alcohol, plus photoassisted formation of acetaldehyde and ethylene were the main flash-initiated processes. Results are briefly compared with various models of the flash illuminated gas/oxide interface