A study of the crystal chemistry of the Ni2-xMn(x)P2O7 systems in powder form, 0less-than-or-equal-toxless-than-or-equal-to2, has revealed the existence of three solid solutions. When x<0.10, the solution is isostructural with low temperature form, alpha-Ni2P2O7. When 0.10 less-than-or-equal-toxless-than-or-equal-to0.40, the structure of the solid solution is similar to that of sigma-Ni2P2O7. When 0.60 less-than-or-equal-toxless-than-or-equal-to2, a form similar to beta-Ni2P2O7 (or Mn2P2O7) is seen. The single crystal structure of material corresponding to x=0.816 has been determined in monoclinic space group Cm, a = 6.557(3), b = 8.738(5), c = 4.498(3), beta = 103.24(1)-degree, V = 240.6(2)Angstrom3, Z = 2, R/R(W) = 3.8/5.3% and shows a structural deformation of the parent lattice. The system shows antiferromagnetic interactions at low temperature.