Gas-liquid reactions often occur in chemical application and are determined by mass transfer from gas to liquid phase. To overcome transfer limitations, microreactors are proposed and the studies are currently underway. Two experimental reaction systems are investigated in a falling film microreactor: reactive CO2 and O-3 absorption with low liquid flow rates. The liquid-wetted surface of the microchannels had to be coated with TiO2 in order to obtain an approximately equal distribution of the flow velocity in the individual channels. Numerical simulation gives first insights to gas flow and mass transfer with the aid of heat transfer analogy. Mass transfer coefficients are determined for different gas and liquid flow rates as well under caustic conditions with different pH values. Ozone absorption in cyanide containing solution is affected by cyanide concentration and pH value, but cyanide degradation rate was not dependent on pH. Reaction of ozone with cyanide is faster than reaction of ozone with OH- even at a high pH value of 13.