In collaboration with local election officials, we conducted a randomized field experiment in which postagepaid envelopes were provided to a random sample of 10,000 permanent vote-by-mail (VBM) voters in San Mateo County, California, in advance of the November 2, 2010, general election. We find that the treatment generated statistically significant but unexpected effects: postage-paid envelopes increased the probability that voters cast their ballots in person and decreased the probability that they cast their ballots by mail. These offsetting effects meant that the intervention produced no net change in voter turnout. We find that this pattern of countervailing effects is strongest among voters who frequently voted by mail in the past, those potentially most susceptible to disruptions in routine. Post-election interviews support the idea that the postage-paid envelopes created confusion for some voters. The results suggest that reforms designed to increase turnout by decreasing voting costs may have the unintended effect of disrupting routines. F or decades, scholars have conjectured that voter turnout rates would rise if the costs of voting were reduced. Early research (Key 1949) considered tangible costs, such as poll taxes; later authors (Kelley, Ayres, and Bowen 1967; Rosenstone and Wolfinger 1978) focused primarily on transaction costs, such as the inconvenience of registering to vote well in advance of an election. Recent years have seen a revival of this line of research in the wake of policy innovations designed to make voting more convenient: