We　have　developed　a　sensor　for　4-aminophenylarsonic　acid　(4-APhAA)　by　coating　a　glassy　carbon　electrode　(GCE)　with　a　composite　prepared　from　an　ionic　liquid　and　dahlia-like　carbon　nanohorns　(CNHs).　The　good　electric　conductivity,　large　surface　area　and　high　pore　volume　of　the　CNHs,　and　the　synergistic　action　of　the　ionic　liquid　(which　is　a　good　dispersant　with　excellent　ion　conductivity)　result　in　efficient　electrocatalysis　towards　oxidation　of　4-APhAA.　The　effect　was　investigated　by　various　electrochemical　methods,　and　the　electron　transfer　coefficient,　diffusion　coefficient,　standard　heterogeneous　rate　constant　and　thermodynamic　activation　energy　were　determined.　The　response　range　of　4-APhAA　was　evaluated　using　an　i-t　plot.　If　operated　at　a　working　voltage　of　900　mV　(vs　Ag/AgCl),　the　sensor　responds　to　4-APhAA　over　the　0.5　mu　M　to　3.5　M　concentration　range.