Chemical reactions initiated by laser-induced dielectric breakdown in homogeneous molecular gases are reviewed. Attention is paid to processes induced in gas mixtures prepared according to the expected composition of planetary atmospheres in order to simulate chemical effects of high-energy-density events (e.g. cometary impact and lightning) in these atmospheres. Utilization of a single pulse from a high-power laser system for this purpose is discussed. The particular processes responsible for the chemical action of laser sparks are identified and described in detail.