This study presents a high-performance liquid chromatography (HPLC) method for simultaneous determination of thiamine and riboflavin and the results of its application to a number of edible seaweeds that are sampled in dried form (Himanthalia elongata, Laminaria ochroleuca, Undaria pinnatifida, Palmaria sp., and Porphyra sp.) or as canned food (H. elongata and Saccorhiza polyschides). Samples are prepared by acid and enzymatic hydrolysis. Optimized conditions for reversed-phase HPLC with fluorescence detection are as follow: column, Kromasil 100 C18; column temperature, 35 degrees C; mobile phase, a 72:28 (v/v) mixture of 0.005 M ammonium acetate (pH 6.7)-methanol; and flow rate, 1.35 mL/min. With these conditions, recovery is 95.52% for thiamine and 90.08% for riboflavin, and the method precision (relative standard deviation) is 2.66% for thiamine and 2.21% for riboflavin. On a dry weight basis, thiamine contents range from 0.14 microg/g in dried H. elongata to 2.02 microg/g in dried Porphyra and riboflavin contents from 0.31 microg/g in canned H. elongata to 6.15 microg/g in dried Porphyra.