Development of in situ monitoring techniques of emulsion polymerization reactions is a real current challenge. First, it may help establish relations between the structure, the physicochemical properties and the performance properties of final latexes. Moreover, it brings a better understanding of chemical and physicochemical phenomenon that occur. Easy installation, performance of measurements and abundance of information gathered (from molecular to material scales) make Raman spectroscopy the adequate technique for this kind of issue, as shown in recent studies [1] [2]. The main goal of this work is to demonstrate the ability of this technique to control the polymerization of styrene via macroemulsion process. Firstly we proved that Raman spectroscopy could be a suitable technique for real time in situ conversion monitoring. Indeed, we demonstrated that normalized Raman signal was ideal to answer this demand because independent of optical parameters but sensitive to processes ones. Raman data were correlated with off-line thermogravimetric results for quantifying the conversion of styrene in polystyrene. Then an in-depth study of its raw signal revealed the sensitivity of Raman spectroscopy to the heterogeneity of the reaction medium. Indeed some peaks of interest seemed to be influenced by optical and chemical parameters. Among these parameters, there are for example the depth position of the analyzed volume in the reactor and the number of particles synthesized during the nucleation step. Off-line studies on samples have already been carry out in order to establish links between physical and chemical phenomenon [3]; our in situ studies could bring additional information. Literature Cited [1] M.C. Chevrel et al. Ind. Eng. Chem. Res. 2012, Vol 51, 16151-16156. [2] N. Brun et al. J. Raman Spectrosc. 2013, Vol 44, 909-915. [3] M. van den Brink et al.J. Raman Spectrosc. 2002, Vol 33, 264-272.