  We show that string moduli have axion-philic nature owing to the
model-insensitive derivative interactions arising from the Kaehler potential.
The decay of a modulus into stringy axions occurs without suppression by the
mass of final states. Interestingly, it turns out to hold in general not only
for the scalar partner of the stringy axion but also for any other moduli. The
decay into (pseudo) Nambu-Goldstone bosons (NGBs) also avoids such mass
suppression if the modulus is lighter than or similar in mass to the scalar
partner of the NGB. Such axion-philic nature makes string moduli a natural
source of an observable amount of dark radiation in string compactifications
involving ultralight stringy axions, and possibly in extensions of the Standard
Model that include a cosmologically stable NGB such as the QCD axion. In the
latter case, the fermionic superpartner of the NGB can also contribute to the
dark matter as a feebly interacting massive particle.
