We have clarified the dynamics and mechanism of CH4 dissociative chemisorption processes on a cesiated Pt(111) surface by means of molecular beam scattering techniques. A comparison is made of similar processes on a Pt(111) surface. It is found that the CH4 molecules with enough energy to overcome the activation barrier for dissociation can no longer dissociate on a cesiated surface of Pt(111). Time-of-flight and angular intensity distribution measurements of the reflected CH4 molecules have revealed a similarity in inelastic collision dynamics both on the Pt(111) and the cesiated Pt(111) surfaces. Even a low concentration of the Cs layer as 0.06 of coverage is found to modify the surface electronic structure such that the activation barrier is enhanced as a result of an increased Pauli repulsive potential component in the potential energy surface. It is also found from time-of-flight measurements that direct dissociation is the only pass way and precursor mediated dissociation is not possible both on the Pt(111) and the cesiated Pt(111) surfaces in our experimental conditions.