The present invention relates to a solid phosphoric acid catalyst and a process for conversion of hydrocarbons using a solid phosphoric acid catalyst. The solid phosphoric acid catalyst comprises silicon orthophosphate, and has a silicon orthophosphate to silicon pyrophosphate ratio of at least about 5:1. The total pore volume of the solid phosphoric acid catalyst is at least about 0.17 cm.sup.3 per gram of catalyst, of which at least about 0.15 cm.sup.3 per gram is contributed by pores with diameter of at least about 10,000 .ANG..