A process for the production of o-phenylphenol in the presence of a catal composed of a .gamma.-alumina or silica-alumina support having a BET surface area of 150 - 300 m.sup.2 /g, a pore capacity of 0.50 - 0.85 ml/g and an iron content of at most 0.1 % by weight in terms of Fe.sub.2 O.sub.3 and an alumina content of at least 90 %, the support carrying thereon platinum in an amount of 0.1 - 5.0 % by weight based on the support, iridium in an amount of 0.1 - 0.4 times that of the platinum and an alkali metal oxide in an amount of 0.5 - 8.0 % by weight of the support. o-Phenylphenol is prepared at a high selectivity over a long period of time by supplying onto the catalyst maintained at 300.degree. - 420.degree. C o-cyclohexylphenol or a dehydrodimerized condensate obtained by aldol condensation of cyclohexanone together with gaseous hydrogen in an amount per hour in terms of liquid volume of 0.1 - 1.5 times the volume of the catalyst, and elevating the reaction temperature stepwise or continuously with the lapse of the reaction time, while slowing down, if necessary, the rate of supplying the starting materials, to effect a dehydrogenation reaction.