A photochemical scheme including a detailed description of multiple seattering up to solar zenith angles of 960 has been used to study a number of different datasets. The good agreement of the model with these datasets and the improvement over previous intercomparisons emphasise the importance of both the diffuse radiation field at wavelengths below 310 nm and multiple scattering at solar zenith angles greater than 90o. These features are ignored in some photochemical models but prove to be very important in modelling photochemistry at dawn and dusk.