Tissular oxygen concentration plays a key role during photodynamic therapy (PDT). Therefore, monitoring its local oxygen partial pressure (pO2) may help predict and/or control the outcome of a PDT treatment. The first real-time, in vivo measurements of the pO2 in the chicken egg’s chorioallantoic membrane, using the delayed fluorescence of photoactivable porphyrins (PAPs), including protoporphyrin IX (PpIX), as monitored with a dedicated optical, fiber-based, time-resolved spectrometer, are reported here. The formation of PAPs/PpIX, photosensitizers of extensive clinical use, was induced in the chicken egg’s chorioallantoic membrane (CAM) with aminolevulinic acid. An excellent correlation between the vascular damage induced by PDT and the reduction in tissular pO2 is found. This study suggests that clinical measurement of the pO2 using the PAPs’/PpIX’s delayed fluorescence (DF) may be used to individualize in real time the PDT light dose applied.