Hydrogen chloride, which is a convenient small molecule “acceptor”, was used to study the donor capacity of π-orbitals in a number of ethylenic and acetylenic hydrocarbons. The composition of the molecular addition compounds formed between these hydrocarbons and hydrogen chloride was determined by thermal analysis. The freezing-point diagrams for hydrogen chloride with a number of lower alkene homologues and with several alkyne homologues are reported. The addition compounds formed by the alkenes have HCl:hydrocarbon molecular ratios of 1:1 and 2:1. An exception is 2-methyl-butene-2 which undergoes rapid chemical reaction with HCl even at low temperatures. The HCl-complexes formed by the alkynes have molecular ratios 1:1, 2:1, and 4:1. Addition compounds of this type are not formed by the saturated hydrocarbons ethane and propane and therefore they must be attributed to specific forces of interaction of HCl with π-orbitals of the unsaturated hydrocarbons. It is suggested that weak hydrogen bonds are formed between the acid acceptor and the donor π-orbitals. To account for the 2:1 complex in the alkenes and the 4:1 complex in the alkynes it is necessary to postulate that each π-orbital can accommodate two molecules of HCl.