JHK_s composite colour image from the SOFI data of the 4 observed fields in W51, zoomed in on the clusters, from left to right field I, field II, field III and field IV. The red, green and blue channels are mapped to the K_s, H and J, respectively. The regions selected for the photometric analysis are indicated with white circles. The spectroscopically classified massive stars in each field are indicated with their ID numbers. In field II, also the positions of the O stars and massive YSO from Figuerêdo et al. (2008) and Barbosa et al. (2008) are shown.