In Canada, a significant portion of coiled-tubing (CT) -drilling activity is in sour environments. Industry and regulatory bodies have recognized that there are no adequate: guidelines for underbalanced drilling of wells containing hydro. gen sulfide (H2S). Underbalanced drilling in "critical" sour wells currently is not permitted pending development of such guidelines. The full-length paper discusses results from a new slow-strain-rate-testing (SSRT) method for CT and contains appendices with a detailed discussion of sulfide stress cracking (SSC) in CT.