To develop a novel method for quantifying the fractional concentration (f b) and the exchange rate (k b) of a specific small-linewidth chemical exchange saturation transfer (CEST) solute in the presence of other unknown CEST solutes. Theory and Methods: A simplified R 1ρ model was proposed assuming a small linewidth of the specific solute and a linear approximation of the other solutes' contribution to R 1ρ. Two modes of CEST data acquisition, using various saturation offsets and various saturation powers, were used. The f b and k b of the specific solute could be fitted using the proposed model. In MRI experiments, using either singlesolute or multi-solute phantoms with various creatine concentrations and pHs, the f b and k b values of creatine were calculated for each phantom; the f b and k b values of phosphocreatine in rats' skeletal muscles were also evaluated. Results: The fitted f b value of creatine from the phantoms were in excellent agreement. The fitted k b value of creatine from the phantoms coincides with that from the literature, as do the f b and k b values of phosphocreatine in skeletal muscles. Conclusion: The proposed approach enables us to quantify the f b and k b values of a specific small-linewidth solute in the presence of other unknown solutes. K E Y W O R D S CEST, chemical exchange saturation transfer, creatine, multiple solutes, phosphocreatine, quantitative CEST analysis 1 | INTRODUCTION Chemical exchange saturation transfer (CEST) has the potential to characterize low-concentration solutes with labile, exchangeable protons in an NMR/MRI field. 1-6 The CEST technique provides contrast on the basis of the chemical-exchange (CE) parameters (ie, the fractional concentration f b , exchange rate k b , transverse relaxation time R 2b , and spectral offset) of specific CEST solute(s). The f b of various solutes are given more attention in relevant studies. 7-11 The k b of some solutes are also widely studied, as the microenvironmental properties (eg, pH) could be indirectly reflected. 12-14 How to cite this article: Wang Y, Chen J-F, Li P, Gao J-H. Quantifying the fractional concentrations and exchange rates of small-linewidth CEST agents using the QUCESOP method under multi-solute conditions in MRI signals.