Description: It is important to determine the effects of vegetation on the water chemistry of headwater streams to ensure appropriate water resource management and landscape planning, particularly because vegetation is known to be one of the primary determinants of the chemistry in such streams and is easily altered by silvicultural operations. Previous studies of headwater stream water chemistry have investigated primarily the effects on baseflow. However, the sources and processes involved vary considerably between baseflow and stormflow due to rainfall. Stormflow water is supplied primarily through soil; accordingly, its chemistry is influenced by vegetation. The present study investigates the water chemistry of streams of headwater catchments, six with coniferous plantations and six with evergreen broadleaved forests, under four stormflow events and three times under baseflow conditions. The studied catchments were located in a hilly region in southwestern Japan and covered relatively small areas (0.7–3.6 ha). Inorganic ions, pH, and dissolved organic carbon were analyzed. A higher concentration of dissolved organic carbon and a lower concentration of Cl − were found in the broadleaved catchments compared to the coniferous catchments under stormflow conditions, but no differences were detected under baseflow conditions. For catchments with older forests, the NO 3 − concentration was higher in the coniferous catchments than the broadleaved catchments under stormflow conditions. These results indicate that these three constituents were not diluted during stormflow and that their presence in soil water may be affected by the type of vegetation. The observed increased NO 3 − concentration under stormflow conditions may result in higher loading downstream.