2,3-Dihydro-4H-pyran, DHP, and 2,3-dihydrofuran, DHF, are photopolymerized at ambient temperature with the aid of diphenyl iodonium hexafluorophosphate, Ph 2 I + PF 6 − or triphenyl sulfonium hexafluorophosphate, Ph 3 S + PF 6 − at λ inc = 310 or 340 nm, respectively. DHP is photopolymerized in bulk with the aid of Ph 2 I + PF 6 − and in CH 2 Cl 2 solution with the aid of Ph 3 S + PF 6 − . The average molar mass of the polymer is low and the propagation involves both C C bonds and ring-opening. DHF is photopolymerized in CH 2 Cl 2 solution using Ph 3 S + PF 6 − . The average molar mass of the polymer increases with conversion (living polymerization) and can become high (4 × 10 5 g mol −1 ). The propagation exclusively involves C C bonds. Dark polymerization of DHF occurs at ambient temperature upon the addition of Ph 2 I + PF 6 − , but Ph 3 S + PF 6 − is inert against DHF. DHP does not undergo dark reactions with the two salts.