In an embodiment is provided a method of determining at least one property of a fluid that includes inducing a paramagnetic response from at least one diamagnetic species flowing through a system, the fluid including the at least one diamagnetic species; performing electron paramagnetic resonance (EPR) spectroscopy on at least a portion of the fluid to generate an EPR spectrum; and determining at least one property of the fluid based on the EPR spectrum. In another embodiment is provided a method of determining at least one property of a first fluid that includes introducing an inhibitor composition to a first fluid flowing through a system to form a second fluid; performing EPR spectroscopy on at least a portion of the second fluid to generate an EPR spectrum; and determining at least one property of the second fluid based on the EPR spectrum. Apparatus for determining fluid properties are also provided.