The focus of this study is the reduction of emissions in Gas-Diesel engines. The experimental results of emissions and performance of a Diesel cycle engine (with turbo and intercooler), obtained by a dynamometer, are presented for operation as Gas-Diesel and as original Diesel engine. First, two kinds of operation were tested: Diesel and Gas-Diesel; then, the respective results of emissions and performance of the motor were compared. After, in Gas-Diesel operation, was evaluated the method of the partial restriction of the admission air, to produce a richer mixture with the same quantity of natural gas used before. The obtained results show a notable reduction of the emissions of carbon monoxide and unburned hydrocarbons for the whole range of evaluated operation. On the other hand, the particulate emissions had kept minims at high rates of substitution and the nitrogen oxides levels presented a bit of increase at high engine load. The thermal performance increases when is achieved a determinate substitution rate value. However, the emissions and thermal performance results have a better behavior at low engine loads (below 50% of the full load). Thus, beyond the levels of some emissions are reduced at high loads, in addition, a reduction was found, of partial form, from one of the main problems involving combustion in Gas-Diesel engines: the incomplete burning of poor mixtures in low loads.