We used photochemical modulation spectroscopy to study the primary radical products of oxygen atom-olefin reactions by their optical absorption. In contrast with earlier work done by mass spectrometric sampling we found that the "vinoxy" radical CH2CHO is an important primary product with ethylene and monosubstituted ethylenes. We characterized two electronic transitions of this radical with origins at 347.7 and 1249 nm and estimated the yields of CH2CHO and HCO in the oxygen atom reactions by comparison with reference reactions of unit quantum yield. Two sources of CH2CHO formation were identified: a pressure-independent pathway and a pressure-dependent mechanism which complements the stabilization of epoxides and aldehydes. Mercury-photo-sensitized decomposition of the corresponding epoxides also produces large yields of CH2CHO. Our observations of pressure-independent fragmentation forming CH2CHO link together early observations of a similar mechanism by Cvetanovic and recent low pressure flow and molecular beam experiments by several groups. A mechanism which accounts for the different characteristics of the pressure-independent and pressure-dependent pathways is discussed. © 1981.