Biphenyl and n-alkylbiphenyls substituted in the ortho position were formed from the pyrolysis of unsaturated fatty acids and algal extracts. Alkylbenzenes and alkylbiphenyls were found to be major aromatic pyrolysis products of a polyunsaturated fatty acid (linolenic acid) and an algal extract (Ecklonia radiata). Pyrolysis on charcoal or Pt/C produced identical C5-alkylbiphenyl distributions for both linolenic acid and the algal extract, suggesting that in the algal extract fatty acids are also involved in the formation of these products. The alkylbiphenyls formed in these experiments are all ortho substituted, which is consistent with their formation from the cyclisation, aromatisation and decarboxylation of fatty acids. Sedimentary samples derived from algal sources show high relative abundances of alkylbiphenyls and anomalously high 2-methylbiphenyl. The occurrence of biphenyl and 2-methylbiphenyl in high relative abundance in sedimentary material may indicate an algal origin.