We point out that the equivalent-photon approximation (EPA) for processes with massive spin-1 particles in the final state would have validity in a more restricted kinematic domain than for processes where it is commonly applied, viz., those with spin-1/2 or spin-0 particles in the final state. We obtain the criterion for the validity ofEPA for the two-photon production of a pair of charged, massive, point-like spin-1 particlesV^±, each of massM and with a standard magnetic moment (κ=1). In a process in which one of the photons is real and the other virtual with four-momentumq, the condition for the validity ofEPA is |q²|≪M², in addition to the usual condition |q²|≪W²,W being theV⁺V⁻ invariant mass. In a process in which both photons are virtual (with four-momentaq andq′), our condition is |q²||q′²|W⁴ ≪ 16M⁸, in addition to |q²| ≪M², |q′²| ≪M² and |q²| ≪W², |q′²| ≪W². Even when these extra conditions permitting the use ofEPA are not fulfilled, convenient approximate expressions may still be obtained assuming merely |q²| ≪W² and |q′²| ≪W².

We also discuss how the extra conditions are altered when the vector bosons are incorporated in a spontaneously broken gauge theory. Examples ofW boson production in Weinberg-Salam model are considered for which the condition |q²||q′²|W⁴ ≪ 16M⁸ is shown to be removed.