A technique involving the microencapsulation of high-vapor pressure compounds suitable for electron capture detection is proposed for the rapid, unambiguous identification of oil slicks. Experimental diffusion studies indicate that the tracer will remain within the microcapsules during the lifetime of an oil slick. Small-scale field studies were conducted with 30-micron diam. microcapsules containing Freon 113. Initial tagging densities of around 100 particles per cm3 were adequate to permit identification for periods up to at least a month after the spill. This identification technique allows the possibility of providing over a million distinguishable mixtures from 20 primary compounds. The analytical procedure associated with this technique is also reported.