Spectral surface plasmon resonance (SPR) sensors with absorbance measurement were prepared. Resonant wavelengths \left({\lambda }_R\right) versus effective refractive indexes of the SPR mode were measured with different media in contact with the gold layer. An investigation into the refractive-index sensitivity of the sensor at a fixed angle reveals a linear dependence of {\lambda }_R on the refractive index of the solution \left({\mathit{n}}_c\right) , with \Delta {\lambda }_R/\Delta {\mathit{n}}_c=3553.6\text{\hspace{0.17em} nm} in a small range of 1.333\le {\mathit{n}}_c\le 1.347 . It was observed that the effective refractive index slowly decreases with increasing {\mathit{n}}_c , attributable to wavelength-induced modulation of optical dielectric constant for the gold layer. Adsorption of bromothymol blue (BTB) on the gold layer leads to a redshift of \Delta {\lambda }_R=3.7\text{\hspace{0.17em} nm} , larger than \Delta {\lambda }_R=2.5\text{\hspace{0.17em} nm} induced by myoglobin (Mb) adsorption. On the basis of Fresnel equations, calculations with d\sim 1\text{\hspace{0.17em} nm} and n=1.69 for BTB and \mathit{d}\sim 3\text{\hspace{0.17em} nm} and n=1.40 for Mb also demonstrate that the SPR band shift induced by full-monolayer adsorption of BTB is larger than that for full-monolayer Mb adsorption. The combination of both measured and calculated results suggests that the contribution of the adlayer index of refraction to the sensitivity of the sensor is greater than that of the adlayer thickness.