Efficient photon upconversion (UC) is a promising way to utilize the infrared photons, where upconversion quantum yield (QY) is a key parameter to find the potential application of these UC materials for many applications. Here we are reporting the absolute upconversion QYs for different concentrations of M₂O₂S:Yb³⁺/Er³⁺ (M = Gd, Y, La) in comparison with β-NaYF₄:20%Yb³⁺/2%Er³⁺ for the UC emissions centered at 410, 550, 667 and 822 nm at varying excitation power densities under 980 nm excitation. Concentration dependent upconversion QY analysis has revealed that La₂O₂S:1%Yb³⁺/1%Er³⁺ and Y₂O₂S:1%Yb³⁺/1%Er³⁺ possess almost similar upconversion QY compared to that of 9%Yb³⁺/1%Er³⁺:La₂O₂S. Measurements under identical conditions with respect to the reported most efficient upconverting phosphor NaYF₄:20%Yb³⁺/2%Er³⁺ show that M₂O₂S:Yb³⁺/Er³⁺ yields a higher absolute upconversion QY at lower excitation power densities compared to that of β-NaYF₄:20%Yb³⁺/2%Er³⁺. Furthermore, UC spectral studies and lifetime measurements were performed for a series of M₂O₂S:Yb³⁺/Er³⁺ phosphors to analyze the UC mechanisms in these phosphors. We show that upconversion emission from ⁴F_9/2 is mainly influenced by the longer lifetime of metastable state ⁴I_13/2.