Measurements of carbon monoxide and nonmethane hydrocarbons were made in situ at the Mauna Loa Observatory from September 1991 through August 1992. A distinct seasonal cycle in concentrations was observed for CO and most NMHCs, with higher average concentrations from January through April and much lower average concentrations from mid-May through October. The sources of CO and NMHCs appeared to vary with season and appeared more homogeneous in winter (urban/industrial) and spring (unidentified). The ocean around Hawaii did not appear to be a major source of CO and most NMHCs, but was the major source of ethane and propene. Ethene and propene were always present in free tropospheric air sampled, indicating daily exchange between the free troposphere and the marine boundary layer. Air arriving at MLO during free tropospheric flow periods had seasonally similar photochemical age (approximately 20 days), with no significant fresh source inputs of NMHCs (except ethene and propene) within at least 10 days. The influence of atmospheric dilution processes on atmospheric concentrations was weakest in the spring period, when the influence of chemical loss processes was greatest. Aircraft measurement made during the spring agreed in trends and absolute concentrations with ground-based measurements made at MLO. The temporal trends in concentrations of NMHCs within altitude ranges were similar. These trends varied with altitude and characterized a layered atmosphere with differing chemical composition. Acetone and methyl ethyl ketone were detected in most samples. N-alkyl aldehydes were also detected but may be artifacts formed from organic aerosols collected in the sampling process. ¿ American Geophysical Union 1996