An analytical method for four isomerized a-acids in beer was developed using solid phase extraction (SPE) coupled with high performance liquid chromatography (HPLC). The method for solid phase extraction was systematically optimized. The optimal conditions were determined as follows: 2 mL of acidic methanol as eluting solvent through a Sep-Pak C18 cartridge and the pH value of beer was adjusted to 2.5. Under these conditions, the recoveries of four isomerized a-acids for spiked standards (n=10) were 90.6%-96.4% and the relative standard deviations (RSDs) were generally less than 4%. The detection limits for iso-alpha-acid, rho-iso-alpha-acid, tetrahydro-iso-alpha-acid and hexahydro-iso-alpha-acid were 0.14 mg/L, 0.36 mg/L, 0.33 mg/L and 0.53 mg/L, respectively. The method is sensitive, accurate and suitable for the analysis of isomerized alpha-acids in beer.