Online discourse from a class of 22 students (11 boys and 11 girls) was analysed to assess advances in conceptual understanding and literacy. The students worked over a two-year period (Grades 3-4), during which they contributed notes to an online Knowledge Building environment--Knowledge Forum[R]. Contributions revealed that both boys and girls produced a substantial amount of text and graphics, and that their written texts incorporated an increasing proportion of less-frequent, advanced words, including academic vocabulary and domain-specific words from grade levels higher than their own. Brief accounts of classroom discourse indicate how deep understanding and vocabulary growth mutually support each other in online and offline exchanges. The gender differences that were observed show boys doing slightly better than girls, suggesting that Knowledge Building has the potential to help boys overcome weaknesses in literacy. (Contains 6 figures, 2 tables and 2 footnotes.)