The effect of changing the block lengths of triblock copolymers of poly(ethylene oxide) -polystyrene -poly(ethylene oxide) on the relative viscosity of AOT oil-continuous microemulsions, with cyclohexane as the oil, was investigated. Five different microemulsion compositions were used to have (a) a series of increasing droplet concentration at constant droplet size and (b) a series with increasing droplet size at constant droplet weight fraction. All polymer and microemulsion mixtures showed Newtonian viscosity, and in all cases the presence of a polymer in the microemulsion led to an increase in the viscosity. Results were presented at 40 degreesC, which is above the theta temperature of polystyrene in cyclohexane. With all polymers, the relative viscosity decreased with increasing droplet concentration (at constant droplet size) and, in general, increased with increasing droplet size (at constant droplet weight fraction), although there were some exceptions. Comparing the viscosifying behavior of the different triblock copolymers on the molar scale (moles of polymer/dm(3)) resulted in the emergence of trends that are significantly different from those of previous investigations of triblock copolymers in microemulsions. In particular, we show evidence that increasing the PS block length leads to increased viscosity enhancement. However, at high droplet concentration this increased viscosity enhancement with long PS blocks is lost as the system comes closer to the segregative phase-separation boundary.