The adsorption dynamics of mixed adsorbed films of hexadecylpyridinium chloride (HPC) and sodium dodecyl sulfate (SDS) were investigated through external reflection-absorption FTIR (ERA FTIR) and surface tension measurements, following two different experimental approaches. In the first approach (case 1), solutions of SDS and HPC, at the desired total concentration and composition, were first introduced into the measurement cell, and then, the measurement was started. In the second approach (case 2), the HPC solution, at a given concentration, was first introduced into the cell, and after adsorption equilibrium was reached, a certain volume of concentrated SDS solution (to attain the desired total concentration and composition) was added to the cell, and the measurement was continued. The experiments clearly showed that the adsorption rate, as well as the mechanism, differed significantly between the two approaches.