Pulsed laser polymerization (PLP) in conjunction with size-exclusion chromatography (SEC), both being carried out in aqueous phase, was used to determine propagation rate coefficients, kp, of non-ionized acrylic acid (AA) at temperatures between 2 and 25°C and monomer concentrations, cAA, from 1 to 10 wt.%. The product kp·cAA,local is the primary experimental quantity deduced via the PLP-SEC technique. Assuming cAA,local to be identical to overall monomer concentration, cAA, yields apparent kp values, which upon enhancing cAA, first increase and, after passing through a maximum at around 3 wt.% AA, significantly decrease. A kp value as high as 180 000 L·mol–1·s–1 was determined for 3 wt.% AA at 25°C. The decrease observed toward higher AA concentration is fully consistent with what has been found in a preceding study into kp of non-ionized AA at monomer concentrations of 20 and 40 wt.%. At constant temperature, variations in apparent kp by about a factor of three are seen in the range up to 40 wt.% AA. Discussion of the measured rate data suggests that it is primarily cAA,local which changes as a function of monomer concentration rather than kp. As a consequence of strong hydrogen bonds in between polymer segments, between polymer segments and AA monomer, and between both these species and water, the AA concentration at the radical site may significantly differ from overall cAA. The assignment of the observed changes in apparent kp to cAA,local is supported by PLP-SEC experiments in which appreciable amounts of propionic acid (PA) have been added to aqueous AA solutions. The addition of PA significantly reduces apparent kp. Addition of NaCl to an aqueous solution of AA in its non-ionized form, on the other hand, does not affect apparent kp. Whether the observed changes in kp·cAA,local are entirely due to cAA,local differing from cAA or whether also the “true” propagation rate coefficient varies cannot be safely decided on the basis of the presently available data.