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Now more than ever there is a growing global interest to reduce greenhouse gas (GHG) emissions originating from internal combustion engines. One approach consists in the use of hydrogen instead of fossil fuels. Large bore gas engines for power generation are often fueled by gases with high methane content. Relative to natural gas-fueled engines, the power densities of premixed or port-fuel-injected hydrogen engines are limited due to low volumetric efficiencies and moreover by occurring irregular combustion events (knocking, backfire). The paper presents results from experimental investigations of the impact of different hydrogen substitution rates in natural gas on performance, emissions and operating limits on a single cylinder research engine. The engine is representative for a large bore gas engine for power generation and operates using an open chamber combustion concept with lean mixtures. Essentially, THC, CO2 and CO emissions decrease with rising hydrogen content of the fuel gas. Even with low concentrations of hydrogen in the fuel gas, significant reductions in THC emissions could be demonstrated. Usually NOX emissions will rise with unchanged operating parameters. However, if excess-air ratio and spark timing are adjusted, a net reduction of NOX emissions can be achieved while the impact on brake thermal efficiency is small. Furthermore, the paper outlines potential mitigation strategies to expand the operational limits with respect to power density with high hydrogen substitution rates.