Propagation rate coefficients, kp, of N-vinyl pyrrolidone (NVP) radical polymerization in aqueous solution have been measured via the pulsed-laser polymerization - size exclusion chromatography method (PLP−SEC) over an extended concentration range from 1.8 to 100 wt % NVP at temperatures between 2 and 60 °C. The SEC analyses have been carried out by a modified procedure using dimethyl acetamide as the eluent. An about 20-fold increase of kp is observed in passing from NVP bulk polymerization to reaction in dilute aqueous solution. As with nonionized methacrylic acid (MAA), for which a similarly strong change in kp has recently been reported, the large solvent effect in NVP polymerization is assigned to intermolecular interactions resulting in a significant hindrance of internal rotational motion in the transition state structure for propagation. Some contribution from a minor change in activation energy may however not be ruled out. PLP−SEC studies carried out on aqueous NVP solutions to which either polyNVP or N-ethyl-2-pyrrolidone (NEP), the saturated analogue of NVP, have been added, demonstrate that kp depends on the molecular environment at the reactive site, which is affected by NVP (or NEP) content, but not by the polyNVP content. The lowering of monomer concentration during NVP polymerization to higher degrees of monomer conversion results in an increase of kp. Variation of pH in the range 3 to 10 does not affect kp.