The implantation of 230 keV carbon ions into alkali halide single crystals AX (A=Na, K, Rb, Cs; X=F, CI, Br, I) at temperatures T ⩽ 77 K gives rise to intensive, highly resolved, optical absorption bands in the visible region (2.3-2.8 eV, peak half width ∼0.004 eV). They resemble very much the Swan band system of matrix isolated C2. The influence of the matrix on the fine structure of the absorption bands is discussed. These carbon-centres are formed to a lesser extent or not at all when implanting at 200 K and 300 K, respectively. They bleach out at room temperature. Radiochemical experiments at room temperature with carbon atoms stemming from the11C(ρ, α)11C-nuclear reaction in BN/AX mixtures show the formation of carbon compounds such as CX4, CX3H, CX2H2, CXH3and CH4. Thus, the primarily formed C2-aggregate or C-X intermediates which are stable at T ⩽ 77 K, at higher temperature are gradually transformed to stable carbon-halogen compounds. The reactions of matrix isolated carbon upon dissolving the ionic crystals are of interest for the labelling of organic compounds with11C