The fairness of eyewitness identi®cation lineups is assessed through the use of a series of fairness measures. All the measures proposed to date are based on the lineup choices of `mock witnesses'. Mock witnesses are persons who have not previously seen the oender but who have been given information about him, usually a verbal description. In a fair lineup mock witness choices should be distributed approximately equally across all lineup members. Simultaneous presentation of lineup members is the most common form of identi®cation procedure, although recent evidence suggests that sequential presentation decreases the proportion of false identi®cations while maintaining the proportion of correct identi®cations. The present study used lineups from a recent court case to explore the limits of the mock witness paradigm using dierent photographs of the defendant, variations in the verbal description, and two presentation modes. Results indicated that suspect choices and the distribution of foil choices were sensitive to changes in the suspect photograph and alternate forms of the verbal description. The patterns of mock witness choices were similar across presentation modes. Our ®ndings raise questions concerning the limits of the validity of the mock witness paradigm. Speci®cally, they illuminate the cognitive process dierences between eyewitnesses and mock witnesses, and the information available for the identi®cation decision.