The possibility to detect and identify early stage membrane fouling in real-time is a key to understand fouling phenomena. In this study normal Raman spectrocopy was explored as a novel online tool for monitoring membrane fouling. Raman spectra were measured during cross-flow experiments in which vanillin was used with different concentrations as a model compound for organic foulants. The measured Raman spectra were analyzed and interpreted by using principal component analysis and external calibration in order to determine the adsorbed amount of vanillin during the fouling process. Fouling induced changes in pure water permeabilities and off-line measured MR spectra were also analyzed in order to confirm the results of the real-time monitoring system. It was found that an increase in the solution concentration increased the degree of vanillin adsorption and led to better water permeabilities. The procedure used is able to provide results that describe well the accumulation of foulants in a straightforward and semi-quantitative way. The results demonstrate that the introduced monitoring tool has potential for gaining new knowledge about fouling layer formation.