Comparability of data across modes is an important issue in survey research. In this paper we discuss item non-response to attitudinal questions in telephone and web surveys. We present results from a survey experiment conducted in Italy and in Spain that compares different presentations of response options in an online setting with a benchmark telephone survey. In line with earlier studies we find that (A), the share of non substantial answers in the online survey depends on how the response option is presented. Comparing different presentations in an online survey to the standard approach of telephone surveys using propensity score matching, we find that (B), the share and pattern of non substantial answers, is most similar across the two modes in the online survey when the it is captured in an instruction on each screen for the online survey. Our findings are of particular relevance for the design of multiple mode or mix-mode surveys of attitudinal questions using online and telephone modes.