Carbon monoxide (CO) photoproduction from particulate and chromophoric dissolved organic matter (CDOM) was determined in seawater from open-ocean and coastal areas. In confirmatory tests, poisoned or non-poisoned filtered and unfiltered blue-water samples, were exposed to sunlight. CO photoproduction was 21–42% higher in the unfiltered than in the filtered samples. In a more thorough study utilizing concentrated particles prepared by 0.2-?m cross-flow filtration, samples containing varying levels of particles were irradiated under simulated solar radiation. Their CO photoproduction rates increased linearly with particle concentration factor. Particulate CO production was 11–35% of CDOM-based CO production. On an absorbed-photons basis, the former was 30–108% more efficient than the latter. This study suggests that in both coastal and blue waters these new-found particulate photoprocesses are of similar biogeochemical importance to the well-known CDOM photoproduction term.