In mixed-mode surveys, it is difficult to separate sample selection differences from mode-effects that can occur when respondents respond in different interview settings. This paper provides a framework for separating mode effects from selection effects by matching very similar respondents from different survey modes using propensity score matching. The answer patterns of the matched respondents are subsequently compared. We show that matching can explain differences in nonresponse and coverage in two Internet samples. When we repeat this procedure for a telephone and Internet sample however, differences persist between the samples after matching. This indicates the occurrence of mode effects in telephone and Internet surveys. Mode effects can be problematic; hence we conclude with a discussion of designs that can be used to explicitly study mode effects.