Fluorescence of the predissocd. C2P(v = 0) state of NO in the presence of rare gases was investigated by using selective excitation with synchrotron radiation. In the presence of Ar an anomalous pressure behavior was obsd.: at low pressures (0-30 torr), the fluorescence intensity was quenched, but at higher pressures (30-400 torr) an enhancement of intensity was obsd. Furthermore the C(0)-A(3) cascade was induced. Similar results are reported for the other rare gases except for Xe, for which only quenching was obsd. It is suggested that the intensity enhancement is due to a decrease in the no. of mols. undergoing dissocn. [on SciFinder (R)]