Within the Emission and Chemical Transformation of Biogenic Volatile Organic Compounds (ECHO) project the photochemistry of biogenic organic compounds in a forest was investigated. In this context, two spectroradiometers were used to determine solar spectral actinic flux and photolysis frequencies within and above a deciduous forest at J¿lich, Germany. Locations in the forest and in a small clearing were examined 2 m above ground. Under overcast conditions, photolysis frequencies were within 1--3% and 6--8% of above canopy values at forest and clearing locations, respectively. In a spectral range below 500 nm the canopy was found to act as a gray filter that diminished spectral actinic flux independent of daytime and wavelength. Under clear-sky conditions, diurnal variations within the forest also followed that of diffuse sky radiation above the forest except for sharp peaks from direct sunlight that was incident sporadically at the selected locations. The results under all conditions were in accord with the foliage being opaque toward UV and gaps in the canopy being responsible for the remaining actinic flux. Sky photographs showed that the distribution and size of gaps in the foliage were extremely heterogeneous with patterns changing rapidly upon changing location. Regarding daytime chemical reactions of biogenic compounds within the forest, it was estimated that degradation by OH was reduced by a factor similar to the photolysis frequencies, while potential degradation by NO3 was increased by a factor of ~3 compared with above forest conditions. As a result, it was estimated that minor fractions of 0.8% of isoprene and 3.4% of monoterpenes were degraded by reactions with OH, NO3, and O3 prior to transport into the layer above the forest.