The light ideality factor determined by measuring the open circuit voltage (VOC) as function of light intensity is often used to identify the dominant recombination mechanism in solar cells. Applying this ‘Suns-VOC’ technique to perovskite cells is problematic since the VOC evolves with time in a way which depends on the previously applied bias (Vpre), bias light intensity, and device architecture/processing. Here we show that the dominant recombination mechanism in two structurally similar CH3NH3PbI3 devices containing either mesoporous Al2O3 or TiO2 layers can be identified from the signature of the transient ideality factor following application of a forward bias, Vpre, to the device in the dark. The transient ideality factor, is measured by monitoring the evolution of VOC as a function of time at different light intensities. The initial values of ideality found using this technique were consistent with estimates of ideality factor obtained from measurements of photoluminescence vs light intensity and electroluminescence vs current density. Time-dependent simulations of the measurement on modelled devices, which include the effects of mobile ionic charge, reveal that this initial value can be correlated to an existing zero-dimensional model while steady-state values must be analysed taking into account the homogeneity of carrier populations throughout the absorber layer. The analysis shows that Shockley Read Hall (SRH) recombination through deep traps at the charge collection interfaces is dominant in both architectures of measured device. Using transient photovoltage measurements directly following illumination on bifacial devices we further show that the perovskite/electron transport layer interface extends throughout the mesoporous TiO2 layer, consistent with a transient ideality signature corresponding to SRH recombination in the bulk of the film. This method will be useful for identifying performance bottlenecks in new variants of perovskite and other mixed ionic-electronic conducting absorber-based solar cell.