We report the hole transport properties of semiconducting polymers in contact with ionic liquids as a function of electrochemical potential and charge carrier density. The conductivities of four different polymer semiconductors including the benchmark material poly(3-hexylthiophene) (P3HT) were controlled by electrochemical gating (doping) in a transistor geometry. Use of ionic liquid electrolytes in these experiments allows high carrier densities of order 10 21 cm -3 to be obtained in the polymer semiconductors and also facilitates variable temperature transport measurements. Importantly, all four polymers displayed a nonmonotonic dependence of the conductivity on carrier concentration. For example, for P3HT in contact with the ionic liquid 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([EMI][FAP]), the hole conductivity reached a maximum of 85 S/cm at 6 × 10 20 holes cm -3 or 0.16 holes per thiophene ring. Further increases in charge density up to 0.35 holes per ring produced a reversible drop in film conductivity. The reversible decrease in conductivity is due to a carrier density dependent hole mobility, which reaches 0.80 ± 0.08 cm 2 V -1 s -1 near the conductivity peak. The conductivity behavior was qualitatively independent of the type of ionic liquid in contact with the polymer semiconductor though there were quantitative differences in the current versus gate voltage characteristics. Temperature dependent measurements of the mobility in P3HT revealed that it is activated over the range 250-350 K. Both the pre-exponential coefficient μ 0 and the activation energy E A depend nonmonotonically on carrier density with E A becoming as small as 20 meV at the conductivity peak. Overall, the peak in conductivity versus carrier density appears to be a general result for polymer semiconductors gated with ionic liquids.