This experimental study investigates the palladium/rhodium based three-way catalytic converter (TWC) in a H2 - gasoline dual-fuel spark ignition (SI) engine under stoichiometric and lean conditions. The work focused on lean-burn engine operating conditions with the aim of reducing NOX emissions during the combustion process, where the TWC is not effective, while improving the thermal efficiency of the engine. Under these lean-burn engine conditions, the combustion promoting properties of H2 allowed for maintained engine combustion stability as determined by the COVimep values even up to ultra lean conditions (λ=2.0). It was found that by reducing the combustion temperature through the application of lean conditions, engine out NOX emissions could be reduced or even eliminated, while under these conditions the TWC was effective in reducing engine-out carbon-based gaseous emissions.