Interaction between p-toluenesulfonic acid (pTSA) and water is studied at -20 degrees C in a CCl4 matrix. In CCl4 water exists as monomers with restricted rotational motion about its symmetry axis. Additionally, CCl4 is transparent in the hydrogen-bonded region; CCl4 thus constitutes an excellent ambient thermal energy matrix isolation medium for diagnosing interactions with water. Introducing pTSA-nH(2)O gives rise to two narrow resonances at 3642 cm(-1) and at 2835 cm(-1) plus a broad 3000-3550 cm(-1) absorption. In addition, negative monomer symmetric and asymmetric stretch features relative to nominally dry CCl4 indicate that fewer water monomers exist in the cooled (-20 degrees C) acid solution than in room-temperature anhydrous CCl4. The negative peaks along with the broad absorption band indicate that water monomers are incorporated into clusters, The 3642 cm(-1) resonance is assigned to the OH-pi-interaction with a cluster containing many water molecules per acid molecule. The 2835 cm(-1) resonance is assigned to the (S-)O-H stretch of pTSA-dihydrate. The coexistence of these two species provides insights into interactions in this acid-water CCl4 system.