Coastal monthly carbonate chemistry data and other associated physico-chemical parameters from Algoa Bay, on the temperate south coast of South Africa. Eight offshore and five inshore sites were sampled. Temperature, salinity and dissolved oxygen were measured at each offshore site, and temperature was measured at each inshore site. At each offshore and inshore site, a single sea water sample was collected at the surface for later analysis of pH and total alkalinity (TA) in the laboratory. Remaining parameters of the carbonate system (including pCO2) were calculated in the software program CO2SYS (Lewis and Wallace, 1998) using pH and TA as input parameters. Temperature, salinity, silicate and phosphate concentrations were included in the calculations as supplemental data which may influence carbonate speciation.