Abstract Microcrystallites of the series of YBa 2Cu 3O x superconductors with oxygen doping in the range 6.44≤x6.98 have been studied by Raman spectroscopy at low temperatures. The abrupt softening of the O(2,3) in-phase phonon above optimum doping, previously reported at room temperature persists to the lowest temperatures studied (∼5K). The dependence of the linewidth of this mode on the oxygen concentration at low temperatures agrees, in general, with the already known room temperature dependence, except at the optimum doping where the phonon remains very wide, an indication of a phase transition. The temperature dependence of the phonon characteristics of the in-phase plane oxygen mode indicate an anharmonic behaviour which increases with decreasing x, in agreement with previously reported data at room temperature obtained with oxygen isotopic substitution. A similar anharmonicity but to a lesser degree is also observed for the apical phonon.