Observed vibrational levels of the D′2Πr state of the PO molecule at 48 835.4 and 48 854.4 cm−1 corresponding to P16O and P18O are shown to have vibrational numbering ν and ν + 1 respectively, where ν is shown to be large (≈21). From this, T0 of D′2Πr is predicted to be about 38 000 cm−1