Item nonresponse in self-administered modes such as Web and mail can be a major problem affecting survey data quality and, in some cases, may be as severe as unit nonresponse. Moreover, little is known about the determinants of item nonresponse in Web and mail household surveys. In this paper, we assess item nonresponse differences by Web and mail modes, question types (e.g. factual, attitudinal, behavioral) and formats (e.g. nominal, ordinal, multi-item, open-end, etc.), and respondent demographics (e.g. gender, age, education, race, and income) in three general public household surveys. For the three surveys, each conducted in the northwestern U.S. in 2007, 2008, and 2009, respectively, we used address-based sampling with the U.S. Postal Service‟s Delivery Sequence File and employed postal mail methods to send all contacts.  Sampled respondents in each survey were presented with a) a mail-only response option, b) a mail response option with a Web follow-up sent two weeks later (i.e. mail+web), or c) a Web response option with a mail follow-up sent two weeks later (i.e. web+mail). The Web and mail questionnaires in each survey were designed very similarly in order to minimize and control for effects from visual design and layout. This paper serves to quantify and describe item nonresponse differences and the sources of those differences, and to identify potential ways of reducing item nonresponse in Web and mail modes of data collection.