A double modulation microwave spectrograph complete with phase sensitive detection and frequency standard has been set up. Widths of the OCS J1-2 line in the ground vibrational state at v=24325.92 MHz as broadened by six different non-polar perturbers, C6H6, CCl4, CS2, C7H16, C6H12 and C6H14, have been measured. The linewidths have been calculated theoretically on the basis of the theory of Anderson and of Murphy and Boggs and compared with the measured values.