Hydrophobic interaction chromatography (HIC) is the method of choice for determination of the drug-to-antibody ratio (DAR) and drug load distribution for cysteine (Cys)-linked antibody-drug conjugates (ADCs). The drug-loaded species are resolved based on the increasing hydrophobicity with the least hydrophobic, unconjugated form eluting first and the most hydrophobic, 8-drug form eluting last. The area percentage of a peak represents the relative distribution of the particular drug-loaded ADC species. The weighted average DAR is then calculated using the percentage peak area information and the drug load numbers. Reversed phase high-performance liquid chromatography (RP-HPLC) offers an orthogonal method to obtain DAR for Cys-linked ADCs. The method involves, first, a reduction reaction to completely dissociate the heavy and light chains of the ADC, then separation of the light and heavy chains and their corresponding drug-loaded forms on an RP column. The percentage peak area from integration of the light chain and heavy chain peaks, combined with the assigned drug load for each peak, is used to calculate the weighted average DAR.