The influence of p-acetoxyacetanilide (PAA) as an additive in the surrounding paracetamol solution was investigated during growth and dissolution of paracetamol crystals. The different crystal faces of paracetamol, {110}, {20 (1) over bar}, and {001}, exhibited two different mechanisms of interaction with the additive. According to the first mechanism, PAA molecules are adsorbed onto the crystal surfaces, i.e. {110} faces, and decrease both the step free energy and the step velocity functions. The second mechanism, which is characteristic for the {001} crystal faces, consists of a selective adsorption of PAA onto the active dislocation sites and suppression of more powerful growth centers in favor of elemental dislocations. The character of adsorption is defined by the affinity between the molecular structure of the crystal surface and the additive molecules, which permits incorporation into the {110} growth sectors. Lattice strain produced by incorporation of PAA exhibits a maximum followed by a relaxation process. This relaxation is caused by formation of cleavage defects and corresponds to an increase of the enthalpy of fusion and decrease of the crystal density as functions of PAA concentration.