The internal friction of lead-indium and lead-tin binary alloy single crystals has been measured as a function of amplitude, temperature and composition in the lead-rich solid solution phase, at about 4 Kc/s. The crystal specimens were electrostatically excited into longitudinal resonance and the resultant amplitude of vibration detected by a frequency modulation technique. The results are discussed in terms of Granato and Lucke's dislocation damping model. The value of the solute-dislocation binding energy deduced from the amplitude-dependent damping is in experimental agreement with that deduced from the amplitude-independent damping, but this value is about four times that calculated from Cottrell's expression. The predicted dependence over a limited composition range of the amplitude-independent damping on the fourth power of the solute concentration has been observed.