Twenty students who earned A or B course-grades in the proof unit(s) of a secondary course that addressed proof in geometry were asked to work on two proof tasks while sharing their thinking aloud and using smartpens. Students were classified into two categories: those who were successful with both proofs and those who were unsuccessful with both proofs. Large differences were observed in how often students in the two groups exhibited certain competencies. The largest gaps occurred in the ways that students: attended to the proof assumptions; used warrants in their proofs; and demonstrated logical reasoning. [For the complete proceedings, see ED629884.]