This work extends the method to follow in real time the reduction of the clay particle size by light extinction measurements (turbidity), obtained via an in-line optical detector fitted at the extruder die exit, to monitor multiple extrusions of PP/MMT nanocomposites. The optical detector was calibrated and showed to follow the Mie Theory. This theory states that the transmitted light intensity decreases for suspension of particles smaller than 500 nm. Corroborating with these results, off-line color measurements has shown that the luminosity decrease and the yellow color increases during multiple extrusions of the nanocomposites. In addition, the color difference and the yellowness indexes of the nanocomposites using organic modified montmorillonite (MMT) are higher then the MMT Na+ (sodium MMT) after multiple extrusions. The addition of clay to a PP matrix produced an increase in the opacity. Results from X-ray do follow the same trend as those obtained from the in-line light extinction measurements. POLYM. ENG. SCI., 2012. (C) 2012 Society of Plastics Engineers