Desalination of brines produced from oil and gas fields is a potential resource for potable water in the southwestern U.S. and other semi-arid areas. Oilfield brines typically contain high concentrations of Na+ and Cl-, as well as other multivalent cations and anions that can reduce efficacy of reverse osmosis (RO). RO separation of NaCl solutions at existence of other co-ions or at increased ion concentration and operating pressure was experimentally tested on a pure silicalite zeolite membrane. Water fluxes increase linearly with increase of net driving force (Delta P-Sigma r(i)RTC(c,f)). Ion flux is essentially independent of the operating pressure but increases exponentially by a power factor of 1.13 as feed concentration increases from 0.001 M to 0.3 M. Inorganic cations (K+, Ca2+, and Al3+) and anions (Br-, and SO42-), when present in a 0.01 M NaCl Solution, play an important role on separation performance of the NaCl Solutions. A notable variation of water flux and ion rejection in the presence of counter cations was observed, while the permeation behavior of 0.01 M NaCl Solution was hardly affected by counter anions. This study is part of a broader project seeking to make desalination of oilfield-produced waters economically viable.