Stored air samples collected at Cape Grim, Tasmania since 1978, have been analyzed by GC-MS to investigate the change in the tropospheric concentration of HCFC-142b and HCFC-141b over the past 15 years. Between April 1978 and September 1993 the concentration of HCFC-142b rose from 0.20¿0.02 pptv to 3.0¿0.3 pptv, with the majority of this increase occurring from 1989 onwards. By mid-1993 the rate of increase had reached 0.9¿0.1 pptv/yr. From 1982 to 1991 the average concentration of HCFC-141b was 0.08¿0.1 pptv. A very sharp increase in concentration began in 1992 however, such that it reached 0.46¿0.05 pptv by September 1993. Using a 2-D model of atmospheric chemistry, constrained by these observations, global emissions of these two compounds have been calculated and found to differ from other estimates. This is partly due to the background concentration levels which existed prior to the start of the other estimated emissions. ¿ American Geophysical Union 1995