Chloroform-soluble palmitic and decanoic acid esters of cellulose were synthesized from the reaction of MCC with acid chlorides in LiCl/DMA and the ionic liquid [amim]Cl, as novel cellulose solvents. A process of derivatization of the remaining hydroxyl groups, as phosphite esters and subsequent P NMR analysis, allowed for simultaneous degree of substitution (DS) determination and quantification of the aqueous-quench acid by-product impurity, after the appropriate calculation. The full mathematical treatment for DS determination is presented, including scripts for the Python and Java programming languages for rapid interpretation of results. This method for DS determination was validated against traditional analyses of the palmitoyl cellulose and the fully substituted p-nitrobenzoyl palmitoyl diester product, from additional reaction with p-nitrobenzoyl chloride. DOSY NMR and SCORE analyses were also employed to demonstrate the utility of this rapid P derivatization and analytical process over traditional 1D NMR analyses.