The isotopic composition of Sr in oil field brines from the Mediterranean Coastal Plain was determined in 18 drillholes (13 of which are oil producing wells). The brines are characterized by salinities ranging from 35 to 93 g/l (TDS), Sr from 28 to 350 mg/l, Sr/Ca molar ratios from 0.011 to 0.053 and 87Sr /86Sr ratios from 0.7075 to 0.7090. E & A = 0.7081 ± 0.0004 (2σ). The brines are classified into two groups: (a) Mavqi'im group - brines with relatively high 87Sr /86Sr ratios (0.7087 and 0.7090), sampled from elastics, dolomites and anhydrites of Upper Miocene age.(b) Heletz group - brines with relatively low 87Sr /86Sr ratios (0.7075 to 0.7081), sampled from sandstones and dolomites of Lower Cretaceous age. Equations were derived to show the relations between 87Sr /86Sr ratio of the brines and the processes through which they evolved (dolomitization and dissolution-reprecipitation). It is suggested that both groups of brines originated from Mediterranean evaporated seawater during the Messinian desiccation. The strontium isotope composition of the seawater is reflected in that of both groups of brines, the Mavqi'im group containing the original 87Sr /86Sr ratio (~0.7088). The Heletz group evolved later on, through exchange reactions of those primary brines with a carbonate sequence of Cretaceous age and consequently new 87Sr /86Sr ratios (~0.7078) could have been developed. The effect of the dissolution reprecipitation reactions on the carbonate rocks is estimated to be very slight, only around 1% (by weight).