The development of the interfacial bond strength as a function of bonding conditions has been investigated in two representative semicrystalline thermoplastics, isotactic polypropylene and polyamide 12. If one side of the interface is well above the melting point immediately before contact, more rapid effective bonding is obtained for a given estimated interface temperature than under isothermal conditions. This is discussed in terms of a simple two-parameter model for the critical strain energy release rate associated with crack propagation along the interface, which incorporates the rate of establishment of intimate contact at the interface. The model provides a self-consistent phenomenological description of the time and temperature dependence of the bonding kinetics in polyamide 12 joints, although questions remain regarding the detailed mechanisms of bonding.