Context For many older people with serious illness, medical decisions will be needed at a time when they are incapable of communicating their wishes. Patients often report decisional conflict about medical treatment preferences, they express preferences that are inconsistent with their values and their substitute decision-makers (SDM) report being uncertain of their loved one’s wishes. Tools are needed to so that patients and their SDMs can be better prepared for future decision-making. Objective To evaluate the efficacy of a decision aid (Plan Well Guide; www.planwellguide.com) administered to a patient with their SDM, in increasing the SDM’s engagement in preparation for decision-making. Study Design: Parallel group randomized trial with wait list control group. Study Setting: Outpatient healthcare settings in Ontario, Canada. Population: People age 65 and older with health conditions, and their self-identified SDM. Intervention: With the SDM present, the patient was guided through the Plan Well Guide, an interactive decision aid that helps patients clarify their values, provides information on the outcomes of medical treatment options in the context of serious illness and links values to preferences. Outcome Measures: The SDM’s score on a validated advance care planning engagement survey (measuring behavioral constructs e.g. self-efficacy, readiness for decision-making) after 3 months. Secondary outcomes included the patient’s advance care planning engagement survey score, the patient’s goal-concordant treatment preferences and decisional conflict after 12 weeks, and satisfaction with Plan Well Guide. Results: 70 pairs of patients and SDMs were randomized; 66 pairs were allocated to Early (n=37) or Delayed (n=29) groups. SDMs were on average 66 years of age (Early 66+11; Delayed 66+10), predominantly female (Early 70%; Delayed 66%) and spouses of the patient (Early 71%; Delayed 66%). Many had no prior experience as SDMs (Early 56%; Delayed 46%). Main and secondary efficacy outcomes will be presented. Conclusions: The Plan Well Guide may assist in improving SDM engagement in advance care planning and readiness to enact the role, by preparing for decision-making regarding the use of medical treatments in serious illness.