Much attention has been devoted to the study of the improved oil recovery (IOR) method(s). However, it still remains a challenge to evaluate the reservoir wettability quantitatively using actual core samples. Contact angle is considered as one of the most common methods to measure the preferential affinity of reservoir rocks. The main objectives of this study are to investigate the influence of droplet volume, brine salinity, liquid saturated rocks, oil acid number, and temperature on rock wettability of carbonate reservoir rock using sessile droplet method. Sixteen runs were undertaken using oil droplet volumes of 10, 15, 20, and 25 ml with different brine salinities of zero, 50,000, 100,000, and 150,000 ppm of NaCl, respectively. This has been done to study the effect of droplet volume and salinity on contact angle. In three runs, different crude oils having acid numbers of 0.374, 0.561, and 0.986 mg KOH/g samples were performed to investigate the influence of acid number on the contact angle. Three runs were carried out using brine, crude oil, and polymer solutions to study the effect of liquid-type saturated rock on contact angle. Finally, two runs were used to study the effect of temperature on contact angle. In all experiments, actual rock and crude oil samples were used.Results indicated that there is a specific droplet volume attained at critical water salinity. This critical water salinity is defined as the salinity at which the oil droplet volume has no effect on contact angle. Rock wettability decreases below the critical salinity and increases above it, depending on the droplet volume. The attained results indicate that liquid saturated rock has an important role on measured values of contact angle. Results also show that an increase in acid number of the crude oil decreases the contact angle. Therefore, carbonate oil reservoirs containing oils of low acid number are expected to be more oil-wet than that ones containing oils of higher acid number. The increase of temperature and bath liquid viscosity reduced the measured contact angle.The results of this study developed a new concept of critical salinity and provided better understanding of some factors affecting wettability measurements using contact-angle technique. (C) 2004 Elsevier B.V. All rights reserved.