The Pak Panang is a fertile yet environmentally degraded coastal embayment in southern Thailand, which is subject to massive flooding during the northeast monsoon. This study characterizes the seasonal variability of the water chemistry and the concentrations and distributions of dissolved arsenic and arsenic in sediments. In situ monitoring of water parameters and water and sediment sampling were conducted during the dry season in 2001 and the rainy seasons of 2002 and 2003. Determinations were made of dissolved, reactive phosphate, nitrate nitrite, silicate, chlorophyll a, particulate carbon content of suspended particulate matter (SPM), and sediments. Analyses of total dissolved arsenic and total particulate arsenic, and other metals, in the sediments were also carried out. The hydrography of the bay was influenced mainly by fluvial input from the Pak Panang River. The nutrient concentrations in the bay were relatively low in both dry and wet seasons, although numerous discharges contribute to the nitrogen load of the bay. The SPM in the rivers had elevated carbon concentrations, up to 21%, resulting in a mean dissolved oxygen saturation of 54% and mineralization of carbon between the catchment and the bay. The total carbon concentrations in the sediments of the bay were independent of season. Total dissolved arsenic concentrations in the bay ranged from 4 to 53 nmol L⁻¹ during the wet season, whereas in the dry season the mean value was 100 nmol L⁻¹. The surface sediments in the bay had higher total arsenic contents in the wet season (210 ± 64 nmol g⁻¹) compared with the dry season (133 ± 48 nmol g⁻¹), indicating a supply from the mineralized catchment during the monsoon.