2,3-Dihydro-4H-pyran, DHP, and 2,3-dihydrofuran, DHF, are photopolymerized at ambient temperature with the aid of diphenyl iodonium hexafluorophosphate, Ph2I+PF6- or triphenyl sulfonium hexafluorophosphate, Ph3S+PF6- at lambda(inc) = 310 or 340 nm, respectively. DHP is photopolymerized in bulk with the aid of Ph2I+PF6- and in CH2Cl2 solution with the aid of Ph3S+PF6-. The average molar mass of the polymer is low and the propagation involves both C = C bonds and ring-opening. DHF is photopolymerized in CH2Cl2 solution using Ph3S+PF6-. The average molar mass of the polymer increases with conversion (living polymerization) and can become high (4 x 10(5) g mol(-1)). The propagation exclusively involves C = C bonds. Dark polymerization of DHF occurs at ambient temperature upon the addition of Ph2I+PF6-, but Ph3S+PF6- is inert against DHF. DHP does not undergo dark reactions with the two salts.