Purpose Despite the extensive research on the determinants of audit pricing in both public and private settings, there is a lack of research about the differences in audit fees between voluntary audits and mandatory audits. This study addresses this gap. Design/methodology/approach Firstly, a theoretical framework is developed to justify differences in audit pricing between voluntary and mandatory audits. Next, using a sample of Spanish private SMEs1 running from 2009 to 2014, we empirically test whether the fees charged for voluntary audits differ from those charged for mandatory ones. We also examine whether the premium observed among large auditors is persistent in the SME setting, and whether this premium differs depending on whether the audits are voluntary or mandatory. Findings Although a preliminary analysis does not report significant differences in pricing between voluntary and mandatory audits, additional analyses using samples restricted by company size show that voluntary audits are charged with a premium. We observe a premium related to large auditors, and find no significant differences in the audit pricing of Big 4 auditors depending on the mandatory/voluntary nature of the audit, b ut the premium associated with Middle-Tier auditors disappears in the voluntary setting. Originality/value This paper contributes to the previous literature by introducing the examination of differences in audit pricing between voluntary and mandatory audits. As far as we know, this is the first study to examine the differences in audit pricing between voluntary and mandatory audits. It also elaborates on studies on audit pricing in SMEs.