In situ near infrared (NIR) spectroscopy is performed simultaneously with automatic continuous online monitoring of polymerization reactions (ACOMP) during methyl methacrylate polymerization. ACOMP is an absolute technique that furnishes weight average molecular mass Mw, intrinsic viscosity, monomer conversion, and other characteristics, whereas NIR furnishes monomer conversion data via an empirical calibration. An advantage of in situ NIR is that it furnishes immediate information on the conversion in the reactor, whereas ACOMP relies on continuous withdrawal and dilution of a small stream of reactor fluid, so that there is a lag time of several minutes between what ACOMP reports and what is occurring in the reactor. Simultaneous monomer conversion data from in situ NIR and ACOMP, the latter derived from both refractive index and UV absorption, are compared and found to be in good agreement. The evolution of conversion kinetics and Mw generally conform to the predictions of the Quasi-Steady State Approximation. Having established the agreement between the methods, the path is now open for combining NIR with ACOMP to characterize increasingly complex systems, such as copolymerization with two or more monomeric species, that are not feasible by either technique separately.