Chemical short-range order (CSRO) critically underpins the physical properties of complex face-centered cubic (FCC) alloys, yet its reliable identification by electron diffuse scattering is often impeded by thermal diffuse scattering (TDS). This work revisits the kinematic theory of first-order TDS to establish a generalizable connection between characteristic TDS features and phonon dispersion relations in FCC systems. Simulations of pure Ni TDS patterns indicate that the commonly observed $1/3\left\{422\right\}$ and $1/2\left\{311\right\}$ peaks and diffuse streaking in $\left[11\overline{1}\right]$ and $\left[11\overline{2}\right]$ zone axes result from low-frequency transverse acoustic $\left\{111\right\}$ (L-branch) phonons in FCC lattices. We experimentally confirm that these hallmark features persist in complex FCC alloys, including a ${\left(\mathrm{CrCoNi}\right)}_{93}{\mathrm{Al}}_4{\mathrm{Ti}}_2\mathrm{Nb}$ medium-entropy alloy and an Fe-17Cr-3Ni-9Mn-3Cu-0.3N-0.14C austenitic stainless steel, and confirm their TDS origin through temperature-dependent measurements. We also discuss how TDS can confound CSRO characterization at the $1/3\left\{422\right\}$ and $1/2\left\{311\right\}$ positions in complex FCC alloys and propose practical strategies for robust CSRO analysis: targeting reflections in orientations where TDS and CSRO peaks separate in reciprocal space, such as the $1/3\left\{511\right\}$ positions in $\left[11\overline{4}\right]$. These results provide a transferable, physics-based approach for interpreting electron diffuse scattering in FCC metals and complex alloys and for designing diffraction measurements that isolate CSRO from ubiquitous TDS.

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• Acoustic phonons
• Face-centered cubic
• Electron diffraction