Poly(spiroorthocarbonate) (PSOC) prepared from pentaerythritol and tetraethylorthocarbonate has poor solubility because of the highly crystalline structure. We synthesized soluble PSOCs having alkyl and phenyl groups by polycondensation of pentaerythritol derivatives with tetraethylorthocarbonate. The structures of the PSOCs were confirmed by NMR spectroscopy and MALDI-TOF mass analysis. Additionally, we examined their application to networked polymers by sulfonium salt-initiated ring-opening polymerisation of epoxy resin with PSOCs to confirm the volume change during crosslinking.