The first measurements of methacrolein (MACR) and methyl vinyl ketone (MVK) in the atmosphere are reported. The compounds both show a strong seasonal variation, with a maximum in the summer. The concentrations of MACR adn MVK are highly correlated with each other and with the concentration of isoprene, in accord with expectation that they are the major initial products of isoprene oxidation. However, the observed MACR/MVK ratio does not agree with the model predictions over the range of conditions normally observed in the nonurban continental boundary layer. We suggest that this indicates a major gap in our understanding of the photochemistry of hydrocarbons in the nonurban troposphere. ¿ American Geophysical Union 1990