The vapour phase isopropylation of biphenyl was carried over modified Y, SAPO and MCM-41 molecular sieves. The conversions of biphenyl were in the range of 31-77% and the selectivities of dialkylated biphenyl were 35-53%. The effect of various reaction parameters like reaction temperature, weight hourly space velocity, molar ratio were studied. The dialkylated products were more over MCM-41 catalysts and the ratio of Sigma monoalkylated/Sigma dialkylated in the products varied with the catalyst and time on stream.