In this work we analyze the possibility of improving the reliability of the conversion from the viscosity function to the molecular weight distribution (MWD) for linear flexible polymers. A constrained inverse procedure has been applied to guide the inverse problem to the correct solution, i.e., an analytical MWD function utilizing the generalized exponential function (GEX) has been assigned. The self-consistency of this flow curve-MWD constrained conversion has been tested on copolyacetal samples of different molecular weights by determining the chi(2) surface in the space of the GEX parameters around the minimum. The result is that the constrained inverse method is capable of yielding meaningful results in terms of MWDs and molecular weight averages provided that the viscosity data fall into the commonly accepted experimental error range and cover an adequately wide shear rate range. Discrepancies between the MWDs calculated from rheological measurements and the corresponding GPC data have been detected in the case of the polymers of lower molecular weight. This event probably occurs because of the lack of viscosity data at very high shear rates due to experimental difficulties. In addition, failure can occur in the rheological model utilized for the lower molecular weight samples.