Measurements of Δ14C, δ13C and δ15N are reported for dissolved (plus colloidal), suspended and sinking particulate, and total sedimentary organic matter in the Santa Monica Basin (mid-basin and shelf sites) on CaBS cruises 1, 3, 4, 7, 8 and 10. These isotopic signatures were indicative of the following processes occurring within the basin regime: (1) terrestrial inputs of organic matter to the sinking and suspended particulate organic matter were of the order of 10% or less, and as high as 25% for the sedimentary organic matter; (2) Δ14C values of the UV-oxidizable dissolved organic matter below 5m were similar to those measured in open ocean waters, while the Δ14C values in the suspended, and, to a lesser degree, in the sinking organic matter decreased markedly with depth. This latter decrease was primarily attributed to episodic resuspension of shelf and slope sedimentary organic matter, and secondarily to natural and/or anthropogenic petroleum inputs; and (3) the isotopic signatures of the UV-oxidizable dissolved organic matter, coupled with total dissolved carbon and amino acid and carbohydrate concentrations were strikingly similar in the deep basin and at an open-ocean site, suggesting a common history for the subsurface (>300m) and deep water at both sites. In addition, total mass and organic carbon and nitrogen fluxes from five particle trap deployments are described in detail. Mass fluxes increased with depth, especially on the shelf, suggesting that particle input from the basin slopes may reach the mid-basin site. We conclude that there is minimal perturbation of all organic phases in the basin from terrestrial sources, and that the properties of the UV-oxidizable dissolved organic matter are not greatly influenced by particles of local origin. © 1992.