Self-assembled monolayers (SAMs) have numerous applications, for example, engine wear inhibitors, surface profiling signal enhancement, nanostructure production, sensor production, and catalysis. The adsorbed SAM structure has a major impact on the properties of the outer monolayer surface which dictates the performance and viability of the SAM for individual applications. Substrate growth phases of SAMs have been extensively studied, and two structures have been identified. Initially, a lying down SAM structure is formed that evolves into a standing up structure. It is often critical to know how both structures form as a function of substrate immersion time to be able to design the properties of this structure. The formation of mercaptopropionic acid (MPA) SAMs on gold has been studied. Electrochemical impedance spectroscopy (EIS) was used to measure the adsorption isotherms at five temperatures in the range 4-40 degrees C. Infrared reflectance absorption spectroscopy (IRRAS) was also used, and the results show close agreement. A new monolayer adsorption isotherm is proposed which models SAM structure formation as a function of immersion time, representing all phases of SAM adsorption. This model represents a significant improvement on previous models based on Langmuir and Kisliuk adsorption isotherms that only model the fractional coverage of a surface with a SAM. The new model predicts the optimum immersion time taken for an MPA monolayer on gold to attain a surface saturated with MPA. It accounts for temperature effects on the rate of formation and the degree of monolayer disorder. It has potential for use in other SAM systems and may become the method of choice for modeling many instances of sequential substrate adsorption of two different structures, each of which exhibits different properties, as a function of immersion time.