Cosolvent effects have historically been considered as negligible at cosolvent spiking concentrations below 1 %. Headspace solid-phase microextraction (HS-SPME) was used to determine the effect of the cosolvent methanol (MeOH) on apparent solubility of three chlorobenzenes (CBs) (1,2,4,5-tetrachlorobenzene, pentachlorobenzene, and hexachlorobenzene). All CBs exhibited a In-linear decreasing relationship between cosolvent volume fraction and extracted mass. However, there were no statistical differences in measured headspace CB concentration at cosolvent additions of 1 % or less compared to controls. This study validated the use of MeOH volumes of 1 % and lower for batch studies using HS-SPME. Ninety-six hour incubation studies using 0.01 %, 10 %, and 100 % MeOH treatments demonstrated chemical losses from headspace vials through time. Declines were greatest for 0.01 % MeOH treatments and nonsignificant for vials containing 100 % MeOH. The silanization of vials using S % DCMCS was unsuccessful in reducing chemical loss; therefore potential loss processes must be further investigated prior to using the HS-SPME method for incubated treatments.