The structure of acenaphtho[1,2-c]-1,2,5-thiadiazole shows that the three independent molecules in the asymmetric unit are approximately planar and that the average bond distances in the thiadiazole ring are: S–N, 1·65 Å CN, 1·31 Å C–C, 1·42 Å the average angle ∠ N–S–N is 99·4°.