A new set of photochromic hindered benzo- and naphtho[1,3]oxazines with fast switching speed between the closed and open form were synthesised. Laser excitation of these uncoloured molecules (with ∼10 ns FWHM pulses) leads to the cleavage of the C-O bond and opening of the oxazine ring in less than 20 ns, generating a thermally unstable coloured zwitterionic species that reverts in few nanoseconds to the initial oxazine closed form. For these derivatives a detailed spectroscopic and photophysical study was undertaken involving measurements of absorption and phosphorescence spectra, phosphorescence quantum yields (ϕ(Ph)), phosphorescence lifetimes (τ(Ph)), triplet energies (E(T)), singlet oxygen (ϕ(Δ)) and ring opening photoreaction (ϕ) quantum yields, reisomerization kinetics and the zwitterionic form lifetime. A remarkable solvent effect on the lifetime of the photogenerated species was observed.