Physical and/or chemical transformations in polymer systems can be studied by different analytical methods. However, most of them are applicable to systems of rather high concentration of polymer, or that takes long periods to prepare samples. In this paper, the Toms effect (drag reduction (DR) in adding very small amounts of a polymer to a solvent if flow takes part in a turbulent regime) is proposed as a method suitable for monitoring chemical reactions of polymers in very dilute solutions (down to tens of ppm). Then, by measuring the Toms effect, there is the possibility of observing slight changes occurring in the course of a reaction taking part in a dilute solution. This publication deals with two typical polymer-polymer transformations - the formation of stereocomplexes between iso- and syndio-tactic poly(methyl methacrylate)s (PMMAs), as an example of physical transition, and denaturating of DNA, as an important chemical reaction.