Knowledge about health is an important factor in the health behavior change process, yet health knowledge is not equally distributed among populations. Research has suggested that differences in health knowledge are based in the influence of social structural and motivational conditions. This study examined socioeconomic status (SES) and other socio-demographic and motivational predictors of diet and health knowledge as part of the formative evaluation of a community-based cancer and diet campaign, the Cancer and Diet Intervention Project. The dependent variable was an open-ended measure of dietary change knowledge. Independent variables included education, income, gender, age a measure of community involvement, and the motivational variables of salience and efficacy for healthy dietary change. Data were collected using a random-digit-dial cross-sectional survey (N=377) of a small midwestern US city (population, 20 000). Findings indicated that response efficacy (belief in personal benefits of dietary change) was the strongest predictor of knowledge about healthy eating, followed by education and gender. Implications of planning public health campaigns are discussed.