The effect of common experimental variables on the surface pressure versus area (pi-A) isotherm of copper tetra-tert-butylphthalocyanine (CuPc(t)) was studied. The influence of temperature and the solvent on the shape and limiting areas of pi-A curves obtained using two troughs, the KSV 5000 balance (Wilhelmy plate method) and the Lauda Langmuir film balance, is reported. The significant impact of experimental conditions and the measuring device on the final isotherm curve explains the wide range of reported values for the limiting area of the CuPc(t) compound and similar Pc derivatives. It is shown that the two measuring devices produce identical results for conventional materials (amphiphiles), where a strong intermolecular interaction at the water-monolayer interface takes place. However, significant differences are observed for nonconventional materials. It is proposed that the degree of divergency between the two measurements of film pressure is in direct correspondence with diminishing intermolecular interactions at the water-monolayer interface.