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<p>We demonstrate room-temperature <span class="inline-formula">¹³</span>C hyperpolarization by dynamic nuclear polarization (DNP) using optically polarized triplet electron spins in two polycrystalline systems: pentacene-doped [carboxyl-<span class="inline-formula">¹³</span>C] benzoic acid and microdiamonds containing nitrogen-vacancy (NV<span class="inline-formula">⁻</span>) centers. For both samples, the integrated solid effect (ISE) is used to polarize the <span class="inline-formula">¹³</span>C spin system in magnetic fields of 350–400 mT. In the benzoic acid sample, the <span class="inline-formula">¹³</span>C spin polarization is enhanced by up to 0.12 % through direct electron-to-<span class="inline-formula">¹³</span>C polarization transfer without performing dynamic <span class="inline-formula">¹</span>H polarization followed by <span class="inline-formula">¹H−¹³C</span> cross-polarization. In addition, the ISE has been successfully applied to polarize naturally abundant <span class="inline-formula">¹³</span>C spins in a microdiamond sample to 0.01 %. To characterize the buildup of the <span class="inline-formula">¹³</span>C polarization, we discuss the efficiencies of direct polarization transfer between the electron and <span class="inline-formula">¹³</span>C spins as well as that of <span class="inline-formula">¹³C−¹³C</span> spin diffusion, examining various parameters which are beneficial or detrimental for successful bulk dynamic <span class="inline-formula">¹³</span>C polarization.</p>