Two mineral oils and 12 linear primary alcohols were studied, alone and in combination, to determine their contact toxicity to adult German cockroaches, Blattella germanica (L.) (Dictyoptera: Blattellidae). The more toxic oil, PD23 (LD₅₀ = 1.45 mg per cockroach) was used for combination studies. Alcohols with carbon chain lengths of C3 and C8 through C12 were the most toxic, with LD₅₀ values ranging from 0.3 to 0.6 mg. C1 (methanol) and C14 (1-tetradecanol) were least toxic, with LD₅₀ values of 2.35 and 1.75 mg, respectively. Eight of the 12 combinations of a nonlethal dose of PD23 oil with an LD₁₀ dose of alcohol produced significantly greater mortality than predicted under the assumption of additive effects. A sample of five synergistic oil alcohol combinations, covering most of the alcohol carbon chain length range over which synergy occurred, was further studied by calculating LD₅₀ values for three fixed mixture ratios (80:20, 50:50, and 20:80) of each combination. Results were analyzed using both graphical techniques (isobole analysis) and by nonlinear regression. At least one, but not necessarily all, of the three fixed ratio combinations of each oil alcohol pairing indicated synergy. The conclusions drawn from the isobole and regression analyses were consistent.