Concentrations of C2--C6 hydrocarbons are reported at Harvard Forest in Central Massachusetts (42.54¿N, 72.18¿; elevation, 340 m), measured at 45-min interval from August 1992 through July 1994. The seasonal cycle of background concentrations for anthropogenic alkanes (ethane, propane, butane, isobutane, pentane, and hexane) and acetylene could be cleanly extracted from the ~900 measurements each month. Background concentrations exhibited seasonal maxima in winter and minima in summer. The phase of the seasonal cycle for each species reflected its rate of reaction with OH: The fast the reaction, the earlier the peak in winter, and the more rapid the decline in spring. Winter:summer ratios for hydrocarbons more reactive than propane were similar, implying a summer:winter ratio for OH of 9¿2 at northern midlatitudes. The annual cycle for ethane provides a measure of the mass-weighted annual mean OH concentration at northern midlatitudes, 7(¿3.5)¿105 cm-3, as simulated by using a three-dimensional chemical tracer model. ¿ American Geophysical Union 1995.