  The NA62 experiment at CERN collected a large sample of charged kaon decays
with a highly efficient trigger for decays into electrons in 2007. The kaon
beam represents a source of tagged neutral pion decays in vacuum. A measurement
of the electromagnetic transition form factor slope of the neutral pion in the
time-like region from $1.05\times10^6$ fully reconstructed $\pi^0$ Dalitz decay
is presented. The limits on dark photon production in $\pi^0$ decays from the
earlier kaon experiment at CERN, NA48/2, are also reported.
