Organic carbon and nitrogen were generally determined with a CHN analyzer after removing carbonate by acidification of samples with liquid acid or HCl vapor. However, some workers have conducted the CHN measurements for coastal marine particles without the acidification and reported their analytical values as POC. Here, we compared the carbon values of the CHN measurements with or without acid treatment (HCl vapor) to suspended, settling particles and sediments in Seto Inland Sea. As a result, the carbon values of samples without acidification were significantly higher than those of untreated ones. Overall, our results indicate the acidification of samples is an indispensable procedure for determining POC in various marine particles.