Experimental evidence is presented that combustion can ignite at room temperature spontaneously inside microbubbles filled with mixture of hydrogen and oxygen. We perform water electrolysis in a closed microchamber by voltage pulses of alternating polarity at repetition frequencies 100 kHz to pump the gases rapidly into the electrolyte and produce extreme supersaturation with both gases. After a delay of 300 600 ls, we observe stroboscopically microbubbles of 5 20 lm in diameter that appear in between the electrodes for several microseconds. Each event is accompanied by a pressure jump of 0:1 1 bar that is measured interferometrically. The pressure jumps are attributed to combustion of the gases in the microbubbles.