By means of surface tension measurement (ring detachment method) mixed micellization and mixed adsorbed film formation were studied between an amphiphilic drug (amitriptyline hydrochloride – AMT) and nonionic surfactants (Tritons and Tweens) at different mole fractions. From the equilibrium surface tension measurements critical micelle concentration (CMC), maximum surface excess (Γ max ) and minimum area per surfactant molecule at the air/solution interface (A min ) were obtained. The theories of Rosen, Rubingh and Maeda were applied to analyse the results. Different thermodynamic parameters, viz. free energy of micellization (ΔG o m ), standard Gibbs energy of adsorption (ΔG o ads ), and minimum energy of surface (G min ) were evaluated. In the mixed micelles, presence of nonionic surfactants between the head groups of drug molecules decreases the repulsion among head groups. As a result, the CMC and Amin decrease while Γ max increases. Interaction parameters of micelles (β m ) and monolayer (β s ) indicate that the drug surfactant systems show better interaction at the interface than in micelles.