With palladium sulphide on barium carbonate as catalyst for dehydrogenation and cycloisomerisation benzo[b]-naphtho[2,1-j]fluoranthene, benzo[a]naphtho[2,3-j]fluoranthene, and certain derivatives of benzo[j]fluoranthene have been synthesised. Sulphuric acid has been found useful for cycloisomerisation without dehydrogenation. The structure of the compound previously claimed to be benzo[a]naphtho[2,1-j]fluoranthene is revised.