With the formic acid--oxygen dosimeter (0.01 M formic acid, 0.001 N sulfuric acid, saturated with oxygen), molecular product, free radical, and water decomposition yields were measured for cyclotron-produced protons, deuterons, and helium ions, and 3.4 Mev alpha particles. Molecular product yields, g(H/sub 2/ ) and g(H/sub 2/O/sub 2/), increase, while the free radical yields, g(OH) and g(H), and the observable yield of-water decomposition, g(--H/sub 2/O), decrease with increasing LET of the incident radiation. However, if it is assumed that the recombination of radicals in the reaction H + OH - H/sub 2/O equals g(H/sub 2/ ) + g(H/sub 2/O/sub 2/), then the total amount of water decomposing into free radicals' g(--H/sub 2/O) T = g(--H/sub 2/O) + g(H/sub 2/) + g(H/sub 2/O/sub 2/), is constant at 4.50 plus or minus 0.13 for all cyclotronproduced particles used. The particles ranged in energy from 20.9 Mev (deuterons) having the lowest LET, to 8.35 Mev (helium ions) having the highest LET. The results are discussed in terms of the free radical model for cylindrical diffusion and are shown to agree well with the one-radical model calculations of Fricke and Flanders. The primary radiation yields, designated by the lower case g(x), are obtained frommore » the experimental yields, G(H/sub 2/), G(H/sub 2/O/sub 2/), G(- O/sub 2/), and G(CO/ sub 2/) by the following series of equations applicable to the formic acid-- oxygen dosimeter: g(H/sub 2/) = G(H/sub 2/); g(H2/sup O/sub 2/) =/ G(H/sub 2/O/ sub 2/)- G(- O/sub 2/); g(H) = 2G(--O/sub 2/) -- G(CO/sub 2/); g(OH) = G(COi). (auth)« less