The performance, emissions and combustion characteristics of lean mixtures of natural gas and hydrogen were studied in a conventional spark ignited engine. Specifically, mixtures of natural gas blended with 5, 10 and 15 percent by volume hydrogen were considered. Engine performance parameters included power (BHP), thermal efficiency (BTE), specific fuel consumption (BSFC), coefficient of variation in mean effective pressure, cumulative energy release schedule, and emissions of CO, NOx and hydrocarbons. Major conclusions of the work include: (i) at equivalence ratios leaner than 0.80, improvements in BHP, BSFC, and BTE were significant with hydrogen addition; (ii) significant extension of the lean operating limit to lower equivalence ratios was demonstrated with increasing hydrogen concentrations in natural gas; (iii) emissions of CO, NO, and hydrocarbons decreased as equivalence ratio was reduced until partial burning became predominant; (iv) hydrogen addition appeared to allow stable engine operation with lower pollutant emissions over a relatively broad range of lean equivalence ratios; and (v) the impact of hydrogen blending on performance and emissions was dependent on the volume fraction of hydrogen, although the functional relationship appeared to be non-linear.