AbstractTrichloroacetic acid petachlorophenyl ester (TCA‐O‐PCP) was found to form a 2:i adduct with benzene. It showed an eight line35Cl NQR spectrum in the range 77 ≦T/K ≦ 312 (decomposition temperature of the adduct). TCA‐O‐PCP was proved to have three crystalline phases. Phase I is stable in the range 77 ≦T/K ≦ 403 (melting point of phase I). It exhibited also an eight line35Cl NQR spectrum. A metastable phase (phase III) was obtained by quenching the liquid TCA‐O‐PCP to 77 K and on heating it was transformed into another metastable phase (phase II) at ca. 310 K. The transition III ⇄ II was enantiotropic and of higher order. AtT/K ≧ ca. 360 a gradual, monotropic transition of II → I was observed. A35Cl NQR spectrum having poor signal to noise ratio was found for phase III. A pronounced rise of the NQR frequency due to steric effect was observed for phase I of TCA‐O‐PCP. Remarkable differences in the characteristics of the NQR spectrum of ‐CCl3group were found between the spectrum of the adduct and that of phase I. Some conformational change of TCA‐O‐PCP molecule accompanied by the adduct formation is thought to be responsible for