The accuracy of interpolation formulas for reproducing the pressure falloff of unimolecular reaction rate constants as given by master equation-RRKM calculations was studied using the CH3 + CH3 --> C2H6 reaction as a test case. It was concluded that the predictive ability of one-parameter formulas is limited to about 20% for a fixed temperature, which degrades substantially for the five formulas tested if the fitting parameter is expressed as a function of temperature over ranges of interest in atmospheric and combustion modeling. A novel two-parameter formula containing a correction term in the denominator of the Lindemann-Hinshelwood expression k/k(infinity) = P-r/[1 + P-r + (P-r/B)(A)] was shown to give better performance than two previously studied three-parameter formulas. When treated on an equal basis (extended to contain three parameters and fitted at each temperature) with the three-parameter equations, which lead to 10% errors, our formula reproduces RRKM rate constants within an accuracy of 3%. With the two fitting constants expressed as temperature polynomials having a total of five parameters, this formula was able to reproduce falloff behavior of RRKM rate constants with a maximum error of 10% from 300 to 2100 K over the pressure range from 10(-5) to 10(5) Torr for both weak and strong collider assumptions.