The kinetics of methoxide ion-catalysed solvolysis of 1-acyl-3-(2-halo-5-nitrophenyl)thioureas and cyclization of fluoro derivatives were studied in methanol at 25 °C. The cyclization involved the substitution of fluorine by sulphur anion of thiourea and proceeded in two steps. With the acetyl derivative, the first step is methanolysis and the second step is much slower cyclization of the 2-fluoro