Abstract The effective fraction of delayed photoneutrons ( β ph) has been theoretically defined and experimentally determined in various different configurations of the LWR-PROTEUS critical facility. The peculiarity lies in the fact that the reactor has D 2O in only one of the four fuelled zones, thus D( γ,n) H reactions take place mainly in this region. The work is divided into three parts. The first part is devoted to the description of the LWR-PROTEUS facility and to the measurements of β ph. These experimental values are derived from standard inverse-kinetics analysis of neutron flux decay experiments for each of seven different configurations, with nine additional groups of neutron precursors to account for photoneutron effects. In the second part, the coupled neutron and gamma Boltzmann equations are reduced to exact point kinetics equations using the photon infinite-velocity approximation, and then to the point reactor model. Photoneutron-specific kinetics parameters, i.e. the photoneutron reactivity ρ ph, the effective photoneutron delayed group fractions β j ph and the photoneutron effectiveness γ ph, are rigorously defined and interpreted. In the third part, ρ ph, β j ph and γ ph values have been estimated for the seven different configurations using only four MCNP4C photonic calculations with unitary photon sources in each of the four fuelled reactor regions. Comparisons have been made with the experiments, and the agreement obtained within ±2 σ between the predicted and measured β ph values is considered remarkably good in view of the simplicity of the models used, the approximations for the adjoint weighting and the complexity of the problem at hand.