A hydrocarbon conversion process is disclosed which may be used to produce high purity isobutylene and/or tertiary butyl alcohol and methyl tertiary butyl ether. A mixed C.sub.4 feed stream is divided into two portions with a first portion being passed through a hydration zone to produce the tertiary butyl alcohol. The remaining hydrocarbons withdrawn from the hydration zone and the second portion of the feed stream are changed to an etherification zone. The unconverted hydrocarbons exiting the etherification zone may be subjected to isomerization and/or dehydrogenation to produce additional isobutylene. The high purity isobutylene is obtained by dehydrating the tertiary butyl alcohol.