Neutron-diffraction　and　magnetization　measurements　have　been　carried　out　on　a　series　of　samples　of　the　magnetorefrigerant　Mn1+yFe1-yP1-xGex.　The　data　reveal　that　the　ferromagnetic　and　paramagnetic　phases　correspond　to　two　very　distinct　crystal　structures,　with　the　magnetic-entropy　change　as　a　function　of　magnetic　field　or　temperature　being　directly　controlled　by　the　phase　fraction　of　this　first-order　transition.　By　tuning　the　physical　properties　of　this　system　we　have　achieved　a　magnetic-entropy　change　[magnetocaloric　effect　(MCE)]　for　the　composition　Mn1.1Fe0.9P0.80Ge0.20　that　has　a　similar　shape　for　both　increasing　and　decreasing　field,　with　the　maximum　MCE　exceeding　74　J/kg　K-substantially　higher　than　the　previous　record.　The　diffraction　results　also　reveal　that　there　is　a　substantial　variation　in　the　Ge　content　in　the　samples　which　causes　a　distribution　of　transition　temperatures　that　reduces　the　MCE.　It　therefore　should　be　possible　to　improve　the　MCE　to　exceed　100　J/kg　K　under　optimal　conditions.