This invention relates to a catalytic composite which is an amorphous solid solution of phosphorus, silicon and aluminum oxides. The composite is characterized in that it contains from about 5 to about 50 weight percent Al.sub.2 O.sub.3, from about 10 to about 90 weight percent SiO.sub.2 and from about 5 to about 40 weight percent P.sub.2 O.sub.5 and has pores whose average diameters range from about 30 to about 200 Angstroms. The composite is further characterized in that it has a pore volume of about 0.35 to about 0.75 cc/g and a surface area of about 200 to about 420 m.sup.2 /g. The composite may be prepared by forming a mixture of sols of alumina and silica and a phosphorus compound, gelling the mixture to form particles and then calcining the particles to provide the amorphous solid solution. The amorphous composite may be used either as is or with additional catalytic metals (selected from the metals of Group VIB and VIII of the Periodic Table) dispersed thereon to catalyze various hydrocarbon conversion processes such as hydrocracking and alkylation.