A study investigated the impact of a procedure called Thesis Analysis and Synthesis Key (TASK), which was embedded in a curriculum designed to help high school students read and write arguments. Subjects, 120 11th-graders in San Francisco were instructed in argumentation under 3 conditions. Their gains in the ability to read arguments were measured with adaptations of the Ennis-Weir Essay Test of Critical Thinking; their gains in the ability to write arguments were measured by holistic scores. Results indicated that in reading arguments, students who received instruction in TASK demonstrated statistically significant improvement. In writing arguments, only those students given TASK in a cooperative learning environment made significant gains in holistic scores when direct comparisons of treatments were made. However, the pre-to-post holistic scores of students in both TASK conditions showed significant gains. In both the reading and writing of arguments, low achieving readers receiving TASK made significantly greater gains than their low achieving counterparts in the control treatment, indicating that facilitators like TASK help students to improve in both the reading and writing of arguments. (Ten tables of data and a figure are included; an appendix contains the thesis analysis and synthesis key.) (Author/PRA)