Pulsed laser polymerization (PLP) in conjunction with size-exclusion chromatography (SEC), both being carried out in the aqueous phase, was used to determine propagation rate coefficients, kp, of nonionized acrylic acid (AA) at temperatures between 2 and 25 °C and monomer concentrations, cAA, from 1 to 10 wt %. The product kpcAA,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 nonionized AA at monomer concentrations of 20 and 40 wt %. At constant temperature, variations in apparent kp by about a factor of 3 are seen in the range up to 40 wt % AA. Di...