Areas per surfactant molecule at the liquid/hydrophobic solid (A(LS)) and the liquid/air (A(LA)) interface as a function of the spacer length are reported for cationic gemini surfactants having (CH2)n spacer s. A(LA) increases with increasing spacer length up to 6-8 CH2 groups in the spacer and then levels off. A(LS) values indicate a more closely packed arrangement of the surfactant molecules than that at the liquid/air interface. Comparison of A(LA) and A(LS) values indicates that the surfactant molecules at the liquid/hydrophobic solid interface are almost three times as closely packed as those at the liquid/air interface. A comparison of the experimental values of the area per surfactant molecule at both interfaces was made with those calculated from dimensions of the surfactant molecule in vacuo.