In a routine procedure for collecting human skin spectra using fiber optic diffuse reflectance spectroscopy, the fiber optic probe is put gently in contact with the skin surface. The purpose of this work is to investigate the influence of probe pressure on in-vivo diffuse reflectance spectroscopy measurements performed by a non-trained operator. In addition, we include simulation using diffusion theory that explains the behavior of diffuse reflectance spectra with the increase of probe pressure.