The low-temperature (∼5 K) phonon-assisted relaxation of the 2p 0 state of phosphorus donors in isotopically pure, monocrystalline 28 Si has been studied in the time domain using a pump-probe technique. The lifetime of the 2p 0 state in 28 Si:P is found to be 235 ps, which is 16% larger than the lifetime of a reference Si:P sample with a natural isotope composition. The interaction of the 2p 0 state with intervalley g-type longitudinal acoustic and f -type transverse acoustic phonons determines its lifetime. This interaction, which depends on the homogeneity of the crystal, becomes weaker in 28 Si because of its more perfect crystal lattice compared to natural Si, and this leads to a longer lifetime. The difference between the linewidths of the 1s(A 1 ) → 2p 0 transition in 28 Si:P and natural Si:P is more than a factor of two. It follows that linewidth broadening due to isotopic composition is an inhomogeneous process.