The present invention relates to a process for preparing dioxolane by reacting ethylene glycol with an aqueous solution of formaldehyde in the presence of an acid catalyst. The raw product comprising water, dioxolane and methanol is fed to a phase separation unit (20) wherein an organic extracting agent is present. An organic fraction stream from the phase separation unit is transferred to an upper part of a purification column (30) from which dioxolane is withdrawn as a bottom product while the distillate stream of the purification column is recycled to the phase separation unit. An aqueous fraction stream from the phase separation unit is transferred to a waste water column (40), withdrawing a waste water stream from the bottom of the waste water column, withdrawing a side stream (42) from a stage between the feed stage and the reflux stage of the waste water column and recycling this stream (42) to the phase separation unit, and withdrawing a distillate stream (43) containing methanol from the waste water column.