Semicontinuous measurements of submicron water-soluble organic carbon (WSOC) aerosol were made simultaneously with organic carbon (OC) and elemental carbon (EC) in the Tokyo urban area in winter, summer, and fall 2004. The measurements of WSOC and OC/EC were made every 6 min and 1 hour, respectively, using a particle-into-liquid sampler (PILS) with a total organic carbon (TOC) analyzer and with an EC-OC analyzer using a thermal-optical technique. The PILS and 12-hour integrated filter measurements of WSOC agreed to within 12%. The WSOC mass concentrations and WSOC/OC ratio showed diurnal variations with peaks at 1200--1400 LT in summer and later in the afternoon in winter. On average, the WSOC/OC ratio was 0.20 and 0.35 ¿g C/¿g C for winter and summer/late fall, respectively. The difference in the winter and summer frequency distributions of the WSOC/OC ratio suggests that the sampled air masses in summer and fall were more photochemically processed than those in winter. Secondary organic carbon (SOC) concentrations were estimated using the EC-tracer method. The measured WSOC was highly correlated with the derived SOC (r2 = 0.61--0.79), with WSOC/SOC slopes of 0.67 to 0.75 ¿g C/¿g C for each season. These results suggest that the WSOC and SOC were similar in their chemical characteristics in this study. Water-insoluble organic carbon (WIOC) ( = OC--WSOC) correlated well with EC and CO (r2 = 0.59--0.73). The diurnally averaged WIOC/EC ratios were nearly constant (1.1 ¿ 0.1 ¿g C/¿g C) throughout the study periods, suggesting that motor vehicle emissions were an important source of WIOC. A dominant portion (about 90% or more) of the POC was water-insoluble, consistent with previous studies of POC.