A field experiment was conducted to investigate the effects of economic disincentives on households' use of private cars. Sixty-eight two-adult households who are owners of one or more cars were randomly assigned to three experimental groups and one control group. Both experimental and control groups logged their car trips during one week before, one week during, and one week after a treatment period. Two of the experimental groups were charged approximately 100% of normal cost per kilometre of driving for 2 weeks, the third experimental group was charged the same amount per kilometre of driving for 4 weeks. Aiming at investigating whether or not deliberate planning attenuates the effects of economic disincentives, the households in one of the former and in the latter group were asked to fill out a prospective car logs. The control group was not charged or requested to fill out prospective car logs but was identically treated in other respects. Analyses of the car logs and odometer readings at the end of treatment showed a weak reduction of car use due to economic disincentive.