Johan Bijnens, Ilaria Jemos Determination of Low Energy Constants and testing Chiral Perturbation Theory at Next to Next to Leading Order Abstract We present the results of a search for relations between observables that are independent of the Chiral Pertubation Theory (ChPT) Next-to-Next-to-Leading Order (NNLO) Low-Energy Constants (LECs). We have found some relations between observables in $\pi\pi$, $\pi K$ scattering and $K_{l4}$ decay which have been evaluated numerically using the old fit (fit 10 in [1] of the NLO LECs. LU TP 09-10, arXiv:0904.3705 [hep-ph]