Aims: Bacterial pathogens such as Pseudomonas aeruginosa and Burkholderia pseudomallei are intrinsically resistant to many classes of antibiotics. This is not only due to the poor permeability of their outer membrane but also because of expression of multiple efflux pumps. A promising strategy to minimize the efflux of drugs by these pumps is the use of efflux pump inhibitors (EPIs). In this study, the potential of caffeic acid derivatives as EPIs in P. aeruginosa and B. pseudomallei were evaluated. Methodology and results: The potential of caffeic acid and its derivatives, i.e. chlorogenic acid, caffeic acid phenethyl ester (CAPE) and caffeic acid phenethyl amide (CAPA) to act as EPIs in P. aeruginosa and B. pseudomallei were assessed using the ethidium bromide (EtBr) accumulation and minimum inhibitory concentration (MIC) validation assays. Among the four test compounds, CAPE was found to significantly increased intracellular accumulation of EtBr in both P. aeruginosa and B. pseudomallei. An increase of 21.4% and 16.8% in cell fluorescence, over a 5-min time frame was observed in P. aeruginosa and B. pseudomallei respectively. Combination of CAPE with kanamycin significantly reduced MICs of this aminoglycoside by a factor of 8-fold in P. aeruginosa and 2-fold in B. pseudomallei. Combination of CAPE with gentamicin also led to a reduction of 4-fold MIC value of this antibiotic in B. pseudomallei. Conclusion, significance and impact of study: The in-vitro results suggest that CAPE has the potential to act as an EPI in P. aeruginosa and B. pseudomallei, thus improving the efficacy of aminoglycosides as antimicrobial agents.