During the Mauna Loa Observatory Photochemistry Experiment (MLOPEX), measurements were made of total odd nitrogen (NOy) and the known individual daytime odd-nitrogen species. The individual species measured were NO, NO2, HNO3, particulate NO3-, peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN), methyl nitrate, and >C3 alkyl nitrates. The most abundant component of NOy was nitric acid; its median contribution to NOy in free tropospheric samples was 43%. The large fraction of HNO3 is consistent with the long transport times and photochemical processing of air masses reaching the mid-Pacific site as well as possible stratospheric input of NOy. The median contribution of NOx to NOy in the free troposphere near 3.4 km was ≈14%. PAN and other measured organic nitrates contributed <7% to NOy. The median sum of the individually measured species was 102% in NOy in upslope periods which consist of a mixture of island-modified marine boundary layer and free tropospheric air. This total was 75% of NOy during downslope periods representative of the free troposphere. This shortfall in the odd-nitrogen budget in the free troposphere corresponds to 72 pptv of reactive nitrogen, which is over 2 times media NOx. The NOy shortfall and the composition of NOy appeared to have a regular variation in the free troposphere during the experiment which was related to air mass origin, recycling of odd nitrogen, and loss processes during transport. The presence of an odd-nitrogen deficit in the remote free troposphere suggests that our understanding of the NOy system is incomplete. Unidentified odd-nitrogen species, such as organic nitrates, may be present, but sampling limitations and analytical uncertainties in NOy and individual (NOy)i measurements still restrict our ability to accurately define an NOy budget, especially in remote regions. ¿ American Geophysical Union 1992