A single-cylinder Gasoline Direct Injection Engine (GDI) engine with a centrally mounted spray-guided injection system (150 bar fuel pressure) has been operated with stoichiometric and rich mixtures. The base fuel was 65% iso-octane and 35% toluene; hydrogen was aspirated into a plenum in the induction system, and its equivalence ratios were set to 0, 0.02, 0.05 and 0.1. Ignition timing sweeps were conducted for each operating point.Combustion was speeded up by adding hydrogen as expected. In consequence the MBT ignition advance was reduced, as were cycle-by-cycle variations in combustion. Adding hydrogen led to the expected reduction in IMEP as the engine was operated at a fixed manifold absolute pressure (MAP). An engine model has also been set up using WAVE.Particulate Matter (PM) emissions were measured with a Cambustion DMS500 particle sizer. The effect of ignition timing on the PM emissions was marked - retarding the ignition led to a substantial reduction in the PM number emissions, especially for the nucleation mode. Overall, retarding the ignition could lead to more than an order of magnitude reduction in particle number for stoichiometric combustion. For the MBT ignition timing, adding a hydrogen equivalence ratio of 0.1 led to more than an order of magnitude reduction in both the PM number and mass for stoichiometric and rich (lambda = 0.9) combustion.