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BACKGROUND:The adsorption equilibrium constant of the Langmuir model (K L ; L mol −1 ) has been applied as the standard thermodynamic equilibrium constant, K o Eq , for calculating the thermodynamic parameters (ΔG°, ΔS°, and ΔH°) of an adsorption processes by using the van't Hoff equation. Some authors have (directly and indirectly) applied the constant K RP (L kg −1 ) of the Redlich-Peterson model for such calculations. However, this is an incorrect application because the unit of K RP is not suitable (it is not an equilibrium constant). Its new adsorption equilibrium constant, K e(RP) (L mol −1 ), was revisited based on a RP (L mol −1 ) g . In the literature, there is still uncertainty regarding the application of a RP as K o Eq for calculating the thermodynamic parameters. Therefore, the present study aimed to evaluate the feasibility of applying K e(RP) to calculate thermodynamic parameters using available literature data. The thermodynamic parameters obtained from K e(RP) were compared to those from K L . A case study using a biosorbent for adsorbing methylene blue dye at different temperatures was carried out to re-verify the feasibility. RESULTS:The Redlich-Peterson model is only valid when its exponent is in a strict range (0 ≤ g ≤ 1). The Redlich-Peterson model (68%; 227 observations collected from 52 published papers) describes adsorption equilibrium datasets better than the Langmuir model. The negative ΔG°values obtained based on K e(RP) (11.7-47.6 kJ mol −1 ) were significantly different (p = 2.98 × 10 −12 ) from those on K L (12.2-40.8 kJ mol −1 ). The magnitudes of ΔH°obtained based on K e(RP) were significantly different (P < 0.05) to those on K L ; however, such differences did not affect conclusions drawn on dominant mechanism adsorption (physical or chemical). The magnitude of ΔH°for chemisorption (involved in covalent bonds) is higher than 200 kJ mol −1 . For the case study, the ΔH°(kJ mol −1 ) and ΔS°[J mol −1 × K −1 ] values calculated based on K e(RP) (11.65 and 111.5) were like those on K L (11.34 and 110.4, respectively). CONCLUSION: A new equilibrium constant, K e(RP) (L mol −1 ), of the Redlich-Peterson model can be applied as K o Eq for calculating the thermodynamic parameters (ΔG°, ΔS°, and ΔH°) of an adsorption processes under specific cases (i.e., F, H, and L-shaped adsorption isotherms). Most of the adsorption processes (98%) involve physical adsorption.