Round Robin exercise 2 evaluated detection efficiency and calibration variability of condensation particle counters (CPCs) at 23 nm and 10 nm across eight laboratories using multiple calibration materials including emery oil, soot, silver, and graphite particles. Measured variability was better than 5% at plateau and around 5% at 23 nm. Material differences from soot at 23 nm ranged from -0.04 to 0.41 units depending on the particle type. Preliminary recommendations include continuing with emery oil for continuity, applying generator/CPC type-specific correlation factors determined by manufacturers when different materials are used, and conducting annual validations with the same material.