Repeated cycling of the pH jump for compound 1 from basic (pH 8.3) to acidic (pH 5.2) over 10 cycles also showed good stability (99% recovery/cycle, Fig ). Slightly improved stability at higher pH values confirms the behaviour observed for hydrolytic degradation in previous studies. For application purposes it is also important to know the timeframe of the pH response. A quick pH response is useful when an abrupt change in environment is required, whereas a slow response can be advantageous for processes requiring slower pH changes (i.e. to control kinetic processes like gel formation). We show that merocyanine 1 could achieve pH drops within seconds (< 5 s) under light irradiation whereas the thermal recovery occurred over minutes (< 20 min). As expected, the thermal recovery is more sensitive to temperature than the photoisomerization. At 28 °C the thermal recovery took approximately a third of the time than at 22 °C (7 min vs 18 min).