This study examined whether 7–9-year-old children preattentively build memories of constancy for individual stimulus features, and if these representations are affected by variability of other stimulus features. This was achieved by looking at the mismatch negativity (MMN) event-related potential to a duration deviant occurring in a stimulus environment in which one or two other features constantly changed. Performance data were also collected, to look at the correspondence between the effects of this manipulation on preattentive and intentional deviance detection. MMN data indicated that the children built a preattentive feature-based memory of constancy that was not affected by the number of varying features. In contrast, intentional deviance detection was considerably impaired by the introduction of feature variability. This dissociation is at variance with previous studies that usually report close association between MMN and behavior.