Abstract: Method of performing alkylation or acylation reactions of aromatic substrates under supercritical or near-critical reaction conditions. In particular, a method of performing Friedel-Crafts alkylation or acylation reactions is disclosed under those conditions. Friedel-Crafts reactions may be effected using a heterogeneous catalyst in a continuous flow reactor containing a super-critical or near-critical reaction medium. Selectivity of product formation can be achieved by varying one or more of the temperature, pressure, catalyst, flow rates and also by varying the ratios of aromatic substrate to acylating or alkylating agent.