The unstable gaseous detonation of stoichiometric propane/oxygen mixtures, diluted or not with argon or helium, was investigated at near-limit conditions. The experiments were performed in a 38-mm-diameter, 24.5-m-long tube; diagnostics included: (i) microwave Doppler velocimetry, using a coaxial waveguide, for a continuous record of the local wave velocity along the entire length of the tube, (ii) fast-response piezo-electric pressure transducers for shock pressure profiles and velocity, and (iii) photodiodes for detecting the luminosity of the reaction zone and measuring the mean combustion wave velocity over most of the tube length. By varying the initial pressure of a given explosive mixture or by changing the dilution (0, 50, GO, or 70 mol%) and the nature of the diluent (Ar or I-it), four modes of propagation could be clearly identified: stable detonation, stuttering mode, galloping mode, and fast flame. For each mixture, the limiting initial pressures for each mode were determined. In particular, it was found that the galloping mode could not be obtained at dilutions equal to or above 70% argon and 60% helium molar dilution. It was also found that, for a given dilution, the galloping wave is produced at higher initial pressures for helium as compared tr, argon dilution. For the undiluted mixture, it was found that the features of the galloping wave were independent from the initiation conditions, apart from a delay for the reinitiation of the first peak. For the same mixture, a reduction in initial pressure was found to reduce the period of the galloping wave. Local measurements have shown that the length of the induction zone increases dramatically when the initial pressure is reduced to around 1-2 kPa, which is also the range of limiting upper pressures for the onset of the galloping wave in the mixtures under investigation. These measurements were in agreement with computed induction times, and the latter could be used to explain the influence of dilution and diluent on The onset of the galloping regime.