The invention provides co-solvents for preparing clear solutions of a biocidal agent known as 10, 10'-oxybisphenoxarsine (OBPA). The clear solutions are stable over a relatively broad range of temperatures and have very low Volatile Organic Carbon (VOC) content. They can be included in master batches for the manufacture of plastic materials, such as flexible PVC. The co-solvents of this invention have very low VOC values, do not have troublesome odors, do not cause significant irritation during the high temperature plastic manufacturing process, and are miscible in a range of useful concentrations with plasticizers presently employed by plastics manufacturers. The use of castor oil and/or modified castor oils such as glyceryl ricinoleate, dehydrated castor oil, and certain alkoxylated castor oils as a co-solvent to increase the dissolvable concentration of OBPA, alone or in solution with a plasticizer, was not known or expected.