Surface layer similarity theory is tested using flux, (co-)variance and profile measurements of carbon dioxide, moisture and temperature taken at the Cabauw tower (The Netherlands) during August 2001. The main focus of the study is to determine the flux-gradient relationships (Phi) for the three scalars. In experimental and modeling studies, it is normal to assume the same expression for the three quantities, i.e. Phi_h=Phi_q=Phi_co2. Our main interest is to investigate the validity of this assumption. This topic is of particular interest to CO2 -flux parameterizations implemented in mesoscale models, which are normally determined on the basis of similarity arguments. We selected observations taken during clear days where convective conditions were dominant during the day and very stable conditions characterized the night situation. Additionally, we only processed data with a correlation coefficient close to 1 between temperature and moisture, and -1 between carbon dioxide and temperature. In consequence, we only considered situations, where non-local effects (entrainment or advection) are negligible. Under convective conditions, the variances of the three scalars have similar profiles and they follow a (1/3)-power law. The flux-gradient relationships however differ largely. The temperature and moisture functions (Phi_h and Phi_q) follow relatively well the standard relation (Phi_h=(1-16(z/L))-1/2 .The Phi_co2 values clearly overestimates this relation. A possible explanation for this disagreement lies in the estimation of the CO2 mean concentration gradient. For temperature and moisture, we had at least three measurement sites below 20 meter (at 2, 10 and 20 meter). For carbon dioxide however, we had only two observation points below 20 meter. Within the framework of the RECAB project, aircraft measurement campaigns are planned for January and June 2002. With this data-set we will attempt to improve the estimation of the mean CO2 concentration gradient. It is also planned to extend our study to analyze the (co-) spectra for the three scalars.