Solar absorption spectra recorded over a 24-year time span have been analyzed to retrieve average free tropospheric mixing ratios of carbonyl sulfide (OCS). The measurements were recorded with the Fourier transform spectrometer located in the U.S. National Solar Observatory McMath solar telescope facility on Kitt Peak (altitude 2.09 km, lat. 31.9¿N, long. 111.6¿W), southwest of Tucson, Arizona, and were obtained on 167 days between May 1978 and February 2002, typically at 0.01-cm-1 spectral resolution. A best fit to the time series shows an average mixing ratio of 566 pptv (1 pptv = 10-12 per unit volume) between 2.09 and 10 km, a small but statistically significant long-term decrease equal to (-0.25 ¿ 0.04)% yr-1, 1 sigma, and a seasonal variation with a summer maximum, a winter minimum, and a peak amplitude of (1.3 ¿ 0.4)%, 1 sigma, relative to the mean. Although a statistically significant decline and seasonal variation have been detected, both are exceedingly small. The present results confirm and extend earlier studies showing that the OCS free tropospheric abundance at northern midlatitudes has remained nearly constant over the last decades.