People with serious mental illness (SMI) die on average 10-15 years earlier than those in the general population. Smoking is the strongest risk factor. With more than 1.2 million inpatient stays for schizophrenia and mood disorders per year in the US., initiating smoking cessation treatment in the hospital is an ideal time to engage SMI smokers. Hospitalized patients are required to be abstinent, are available for counseling, and can try cessation medications in a supportive setting. The challenge is how to support SMI smokers to maintain abstinence after they leave the hospital. This ongoing study will adapt EX to address the specific challenges of SMI smokers and evaluate its effectiveness in promoting abstinence following hospital discharge. In Aim 1, we will develop and obtain preliminary feedback about EX-SMI, a version of EX adapted for SMI patients. A priori target mechanisms are motivation, abstinence self-efficacy, and cravings. In Aim 2, we will conduct a one-arm pilot trial and obtain feedback from 20 SMI smokers recruited during psychiatric hospitalization and followed up at 2-weeks and 1-month post-discharge. Feasibility and acceptability will be assessed via participant feedback regarding their intervention experience, changes in motivation, self-efficacy and cravings, and automated tracking data on EX-SMI use post-discharge. In Aim 3, 90 SMI smokers recruited during psychiatric hospitalization will be randomized to usual care or to a revised version of EX-SMI and followed for 3 months after the hospital stay. We will compare the feasibility/acceptability of EX-SMI, measure intervention impact on target mechanisms, and obtain initial estimates of efficacy on 7-day point prevalence abstinence. Ultimately, this program of research could have a powerful public health impact by increasing smoking cessation among persons with SMI.