The photo-stabilizing eftect of 1，2，2，6，6-pentamethyl-4-hydroxy-piperidine (PMP) in isooctane solution．containing hexadecene-1 and di-t-butyl peroxide．has beern investigated．It is shown from the experimental data that the photostabilizing effee-tivenesa of PMP is approximately the same as 2，2，6，6-tetramethyl-4-hydroxy-piperidine (TMP) and slightly higher than that of 2

2，6，6-tetramethyl-4一hydroxy·piperidine-N-oxyl (NO)，and that PMP，as well as TMP，produces N-oxyl in the photostabilizing process of PMP and the concentration of NO produced changes through a maximum-There is a difference related to the production of NO between PMP and TMP．In the ease of PMP the rate of cumulation of NO is slower and the maximum value of NO concentration is lower than that in the ease of TMP．It has been also found in experi-ments that a stable intermediate．1，2，2，6，6．pentamethyl-4-hydroxy．piperidine-N-oxide (PMPO) is produced in photostabil．izing process of PMP．From the above mentioned facts it is suggested that the stabilizing effect of PMP is aeeomplished by a two-step mechanism in which PMP is first transformed into its oxide PMPO．and then PMPO is further transformed into NO．