Participants in this study completed an online experiment in which they wrote essays by stylus or keyboard. Three translation measures (length of language burst, length of pauses, and rate of pausing) and four transcription measures (total words, total time, words/minute, and percent spelling errors) for composition were analyzed for two research aims. Research Aim 1 addressed whether upper elementary and middle school students with carefully diagnosed transcription disabilities (dysgraphia with impaired handwriting, n=18, or dyslexia with impaired spelling, n=20) showed significant differences from pretest to posttest, across modes of transcription (stylus or keyboard), and between diagnostic groups. Results showed significant (a) change after intervention (18 computerized lessons with learning activities in letter formation/selection, spelling, and composing) in length of pauses, total time, and words per minute; (b) mode effects (fewer words and less time by stylus; fewer pauses per minute by keyboard); and (c) interactions with diagnostic group in response to intervention on some measures. Research Aim 2 addressed whether following intervention each of the diagnostic groups performed comparably to a typical control group (n=15) in the same online experiment. Results showed (a) comparable performance of the dysgraphia and control groups on all keyboarding tasks but differences on two stylus measures; and (b) lack of comparable performance of the dyslexia and control groups on two stylus measures (total words and percent spelling errors) and the four keyboarding tasks related to transcription. Implications for assistive technology and writing instruction for dysgraphia and dyslexia are discussed.