Abstract: Phosphorus-based compositions useful as biocides for the control of a broad spectrum of harmful micoorganisms are disclosed. The compositions contain at least one compound of the general formula(RO).sub.2 P(O)CXYZwherein R is H, a C.sub.1-12 aliphatic hydrocarbon radical or a phenyl radical, X is H or halogen, Y is halogen, and Z is a substitutent selected from the group consisting of COOH, CN, COOR and CONR.sub.2, with the proviso that X and Y cannot both be C1. The effective concentrations of the biocides vary according to the specific system, generally being in the range of between 0.1 to 5000 parts per million of substrate. The compositions may be employed alone or in the form of mixtures with other known compatible active agents. The compositions show high biocidal activity against various microorganisms in comparison with known biocides.