Abstract Caper spurge ( Euphorbia lathyris L.) seed oil contains a series of diterpenoids known as Euphorbia factors, or L‐factors, L1–L9. They are esters of several polyols (lathyrol, epoxylathyrol, hydroxylathyrol and ingenol) and account for about 3–5% of the oil. The percentage of ingenol‐based L‐factors is very low, less than 5% of the diterpenoid fraction, but some of them (factors L5 and L6) are responsible for the irritant and co‐carcinogenic activities of the oil. This paper reports an HPLC‐UV and HPLC‐positive‐ESI‐MS analysis of the diterpenoid fraction of caper spurge seed oil before and after selective hydrolysis of ingenol‐based L‐factors. Separation of lathyrane polyols and esters, and ingenol and its esters was achieved using a chromatographic system consisting of a C 18 stationary phase and acetonitrile:water as mobile phase. A new macrocyclic constituent, the deoxy Euphorbia factor L1, was identified in the oil. Copyright © 2001 John Wiley & Sons, Ltd.