Complex interactions between numerous components of essential oils often contribute to the pharmacological effect and therapeutic outcome. To further elucidate these interactions, several essential oil constituents (EOCs) were combined in different ratios and their inhibitory effects on the growth of bacteria and yeast determined using the minimum inhibitory concentration (MIC) microplate assay. When combined and tested against Candida albicans, (+)-β-pinene interacted antagonistically with (-)-menthone (∑FICT = 9.80), but synergistically with 1,8-cineole (∑FICT = 0.35). Against Escherichia coli, the combination of E- and Z-(±)-nerolidol and geranyl acetate displayed an additive interaction (∑FICT = 1.04); while a variable interaction was observed between E- and Z-(±)-nerolidol and eugenol with antagonism and synergy being observed at different ratios of each EOC. The combination of either carvacrol or eugenol with an antimicrobial agent (ciprofloxacin or amphotericin B) resulted in synergistic interactions against all microorganisms tested. These favourable results further support the use of essential oil constituents as adjuvants in the development of a new generation of phytopharmaceuticals that can be used in combination with synthetic drugs against drug-resistant microorganisms.