Adultery in Taiwan’s Criminal Law is defined as sexual intercourse between a married person and someone other than his/her spouse, however, a person cannot be accused of adultery unless substantial evidence of sexual intercourse is found. Until recently, electronically transmitted messages were not considered substantive proof of adultery, but new cases have acknowledged the validity of internet and MSN messages, representing a major change in the direction in Taiwan’s legal practice. The question we raise here is whether internet messages should be considered substantial evidence in current legal principle and practice. In an attempt to address this thorny question, we conduct an analysis of contemporary Taiwanese case law concerning adultery. This study is also aided by the application of probability theories of evidence in logical positivism to illustrate the legal status of internet evidence. Adopting this interdisciplinary approach, this paper aims to better illuminate the pressing problems of whether and how internet evidence can play a significant role in establishing adultery.