Objective Older adults are hospitalised from the emergency department (ED) without potentially needing hospital care. Knowledge about rates, associative factors and causes of these preventable emergency admissions (PEAs) is limited. This study aimed to determine the rates, associative factors and causes for PEAs of older adults. Design A mixed-method observational study. Setting The EDs of two Dutch hospitals. Participants 492 patients aged >70 years and hospitalised from the ED. Measurements Quantitative data were retrospectively extracted from the electronical medical record over a 1-month period. Admissions were classified (non)preventable based on a standardised approach. Univariate and multivariate multilevel logistic regression analyses were performed to identify possible associations between PEAs and demographic, clinical and care process factors. Qualitative data were prospectively collected by email and telephone interviews and analysed thematically to explore hospital physician’s perceived causes for the identified PEAs. Results Of the 492 included cases, 86 (17.5%) were classified as PEA. Patients with a higher age (adjusted OR 1.04, 95% CI 1.01 to 1.08; p=0.04), a low urgency classification (adjusted OR 1.89, 95% CI 1.14 to 3.15; p=0.01), and attending the ED in the weekend (adjusted OR 2.02, 95% CI 1.22 to 3.37; p<0.01) were associated with an increased likelihood of a PEA. 49 physicians were interviewed by telephone and email. Perceived causes for PEAs were related to patient’s attitudes (eg, postponement of medical care at home), provider’s attitudes (eg, deciding for admission after family pressure), health system deficiencies (eg, limited access to community services during out-of-hours and delayed access to inpatient diagnostic resources) and poor communication between primary care and hospital professionals about patient treatment preferences. Conclusions Our findings contribute to existing evidence that many emergency admissions of older adults are preventable, thereby indicating a ...