The mechanism of nitrogen sorption in porous silica was investigated by small-angle neutron scattering (SANS). Two samples of porous silica were studied containing mesopores (pore sizes 5.5 and 9.5 nm, respectively) and additional micropores of irregular shape and statistical distribution. SANS curves were recorded at a temperature of 78 K at various relative pressures pipe during adsorption, The experiment is based on contrast matching between silica and condensed nitrogen with regard to neutron scattering. The sorption process was characterized by the evaluation of the chord-length distributions extracted from SANS data for each pipe. In addition, a general approach was developed to relate the SANS pattern during capillary condensation to the size distribution and the morphology of ordered mesopores. On the basis of these evaluation methods, various uptake mechanisms could be described, which are micropore filling, the formation of nitrogen layers, and capillary condensation. The analysis of the SANS data shows that the mean size of the remaining empty mesopores formally increases, and their size distribution becomes narrower during capillary condensation, which is in agreement with the predictions of the Kelvin equation. Furthermore, our study indicates a significant degree of additional microporosity, the origin of which is discussed. For comparison, the experiment and the data evaluation were also applied to a disordered porous silica with a broad pore size distribution. The combination of SANS and nitrogen sorption turned out to be a powerful technique to investigate both the mechanisms of sorption and the structure of porous silicas in one experiment.