We present a surface plasmon resonance (SPR) sensor for crude Oils that can be Used to directly measure asphaltene deposition onset. Surface plasmon resonance describes a condition in which light incident onto a highly conductive metallic film couples into resonant charge oscillations of the metal. The SPR condition is highly sensitive to Slight perturbations in the dielectric environment in the immediate vicinity of the thin-film, less than 1 mu m away at visible frequencies. Here we show that shifts in the peak surface plasmon resonant wavelength can be used to measure the onset of deposition of asphaltenes from a titration experiment. Initially, the SPR peak wavelength of the neat crude oil is measured. Next, the gradual addition of n-heptane dilutes the crude oil and produces a lower SPR, peak wavelength, which results from the lower refractive index of the mixture. When the amount of added n-heptane approaches the deposition onset point, asphaltenes precipitate and ate deposited onto the thin-film surface. We then observe that the SPR peak wavelength increases as the deposit is formed on the sensing surface. The asphaltenes continue. to deposit on, the surface until a deposit thicker than the SPR field penetration depth is reached. Even though the crude oil has been substantially diluted in n-heptatie, the final SPR peak wavelength exceeds that of the native crude oil due to the dense asphaltene deposit on the surface. Three crude oils were evaluated with the SPR approach for measuring asphaltene deposition onset.