We investigate the short- and long-term impacts of the Federal Reserve's large-scale asset purchases (LSAPs) on non-financial firms' capital structure using a threshold panel ARDL model. To isolate the effects of LSAPs from other macroeconomic conditions, we interact firm- and industry-specific indicators of debt capacity with measures of LSAPs. We find that LSAPs facilitated firms' access to external financing, with both Treasury and MBS purchases having positive effects. Our model also allows us to estimate the time profile of the effects of LSAPs on firm leverage providing robust evidence that they are long-lasting. These effects have a half-life of 4-5 quarters and a mean lag length of about six quarters. Nevertheless, the magnitudes are small, suggesting that LSAPs have contributed only marginally to the rise in U.S. corporate debt ratios of the past decade.