The number of unlisted companies that voluntarily adopt IFRS is increasing among firms not subject to mandatory K-IFRS (Korean International Financial Reporting Standards) application. Despite the burdens associated with IFRS adoption—such as shortened disclosure deadlines, application of complex accounting standards, and implementation costs—these unlisted companies reportedly choose IFRS for reasons including consistency of accounting standards between parent and subsidiary companies, pursuit of public listing, enhancement of corporate credibility and corporate image, and overseas capital procurement. However, our investigation revealed that a considerable number of unlisted companies that adopted K-IFRS at significant cost subsequently disappeared through closure, merger, or transformation into entities that no longer fall within the scope of external audit requirements under the External Audit Act. Specifically, among 1,552 unlisted companies that voluntarily adopted K-IFRS, 1,172 companies (75.5%) continued to apply IFRS, while the remaining approximately 24.5% (380 companies) underwent mergers (122), closures (29), conversion to general companies not subject to external audit (77), and some even reverted to K-GAAP (22). Meanwhile, larger firms, firms audited by Big 4 accounting firms, firms with higher total asset growth rates, and younger firms were more likely to voluntarily adopt international accounting standards. In contrast, the existence of controlling relationships, debt ratios, return on total assets, export ratio, and corporate tax burden were found to be unrelated to the characteristics of firms voluntarily adopting IFRS. Furthermore, to examine changes in financial condition and business performance of unlisted companies that adopted IFRS, we conducted a comparative analysis with matched K-GAAP-applying unlisted companies using financial variables expected to be related to the characteristics of IFRS-adopting unlisted companies. The analysis results showed that IFRS-applying unlisted companies continued to grow in firm size even after IFRS adoption. Additionally, return on total assets (ROA), an indicator of accounting profit, showed a statistically significant tendency to increase compared to matched firms before and after IFRS adoption, although this was not a persistent phenomenon. In contrast, no statistically significant difference in operating cash flows was found between K-IFRS-adopting firms and K-GAAP-applying matched firms after IFRS adoption. Finally, the debt ratios of IFRS-applying unlisted companies increased before and after IFRS adoption, but no statistical significance was found compared to K-GAAP-applying firms. The results of this study are expected to provide useful information for decision-making by unlisted companies preparing for IFRS adoption and related supervisory authorities and other users of accounting information.