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We report observations with the IRAM 30-m telescope of SiO(2-1), (3-2), and (5-4) towards the Orion Bar, S 140, NCC 2023 and NGC 7023. We detect SiO towards the Orion Bar and the S 140 photon dominated region (PDR) with an abundance of order 10 −11 relative to H2. Towards the Orion Bar, SiO is extended and is not confined to a layer close to the ionization front. We put limits of 2-4 × 10 −11 on [SiO]/[H2] towards several positions in NGC 2023 and NGC 7023. In order to interpret these data, wedoi:10.1051/0004-6361:20010470 fatcat:7dktjfvlfbeejgt2gfdsh35bte