Single-walled carbon nanotubes can be selectively doped by conjugated polyelectrolytes (CPEs) to form either p- or n-type composites. The selectivity of charge-transfer doping is found to be dictated by the polarities of CPE pendant ionic functionalities. This finding leads to a fundamentally new approach to both p- and n-type solutionprocessable composites for high performance, flexible thermoelectric devices.