Barium sulfate is one of the most-difficult types of scale to inhibit in oil- and gas-production systems because of its physical hardness and its chemical and thermal stability. Barium sulfate is most commonly inhibited with either phosphonate or polymeric scale inhibitors (SIs) deployed at substoichiometric concentrations. What is less well-known in the oil industry is the effect resulting from the use of combinations of two (or more) SIs synergistically for enhanced scale-inhibition performance. A positive "synergistic" effect would be one in which, for example, 5 ppm of A + 5 ppm of B performed better than 10 ppm of either A or B. In this paper, a series of static barium sulfate inhibition-efficiency (IE) test results are presented, in which a series of pairs of SIs have been tested to determine their synergistic properties at pH 5.5 and 95°C. Polymers can be blended with phosphonates or, alternatively, pairs of phosphonates or polymers may be applied. In all cases, the synergistic IE is compared with the IE of each SI tested independently at the mass dosage (i.e., the same concentration in mg/L or ppm). Each separate single SI used in the work has been tested previously for barium sulfate IE at pH 5.5 and 95°C to determine the minimum inhibitor concentration (MIC) for each species (Shaw et al 2012a, b). Previously, nine phosphonate and nine polymeric SIs were tested individually; in this work, 34 SI combinations have been tested to examine their synergistic properties. The MICs of the synergistic blends are compared with the normal MICs of the individual SIs. Surprisingly, in most cases, the IE of the blends is usually higher over the range of SI concentrations tested (i.e., the MIC of the blend is lower), compared with that of each SI tested separately. Certain "pairs" of SIs used together yield a significantly beneficial effect (e.g., DETPMP and HMTPMP, both pentaphosphonates). Some mechanistic reasons for these synergistic pairs working particularly well are suggested.