  We present a detailed numerical study of the K_{e3} decays to
O(p^6,(m_d-m_u)p^2,e^2p^2) in chiral perturbation theory with virtual photons
and leptons. We describe the extraction of the CKM matrix element |V_{us}| from
the experimental K_{e3} decay parameters. We propose a consistency check of the
K^+_{e3} and K^0_{e3} data that is largely insensitive to the dominating
theoretical uncertainties, in particular the contributions of order p^6. Our
analysis is highly relevant in view of the recent high statistics measurement
of the K^+_{e3} branching ratio by E865 at Brookhaven which does not indicate
any significant deviation from CKM unitarity but rather a discrepancy with the
present K^0_{e3} data.
