diff --git a/sexp.c b/sexp.c
index a9fdccd5..9ed817e6 100644
--- a/sexp.c
+++ b/sexp.c
@@ -1865,7 +1865,7 @@ sexp sexp_write_one (sexp ctx, sexp obj, sexp out) {
   unsigned long len, c;
   long i=0;
 #if SEXP_USE_FLONUMS
-  double f;
+  double f, ftmp;
 #endif
   sexp x, *elts;
   char *str=NULL, numbuf[NUMBUF_LEN];
@@ -1913,7 +1913,10 @@ sexp sexp_write_one (sexp ctx, sexp obj, sexp out) {
       } else
 #endif
       {
-        i = snprintf(numbuf, NUMBUF_LEN, "%.16g", f);
+        i = snprintf(numbuf, NUMBUF_LEN, "%.16lg", f);
+        if (i >= 16 && sscanf(numbuf, "%lg", &ftmp) == 1 && ftmp != f) {
+          i = snprintf(numbuf, NUMBUF_LEN, "%.17lg", f);
+        }
         if (!strchr(numbuf, '.') && !strchr(numbuf, 'e')) {
           numbuf[i++] = '.'; numbuf[i++] = '0'; numbuf[i++] = '\0';
         }
@@ -2434,7 +2437,7 @@ sexp sexp_read_polar_tail (sexp ctx, sexp in, sexp magnitude) {
 sexp sexp_read_float_tail (sexp ctx, sexp in, double whole, int negp) {
   int c, c2;
   sexp exponent=SEXP_VOID;
-  double val=0.0, scale=0.1, e=0.0;
+  long double val=0.0, scale=0.1, e=0.0;
   sexp_gc_var1(res);
   sexp_gc_preserve1(ctx, res);
   for (c=sexp_read_char(ctx, in); sexp_isdigit(c);
@@ -2475,7 +2478,7 @@ sexp sexp_read_float_tail (sexp ctx, sexp in, double whole, int negp) {
 #endif
   }
   if (e != 0.0)
-    val = abs(e) > 300 ? exp(log(val) + e*M_LN10) : val * pow(10, e);
+    val = abs(e) > 320 ? exp(log(val) + e*M_LN10) : val * pow(10, e);
 #if SEXP_USE_FLONUMS
   res = sexp_make_flonum(ctx, val);
 #else
diff --git a/tests/r7rs-tests.scm b/tests/r7rs-tests.scm
index 2585c39d..28bfb437 100644
--- a/tests/r7rs-tests.scm
+++ b/tests/r7rs-tests.scm
@@ -2344,6 +2344,36 @@
 ;;(test-numeric-syntax "#e1.0+1.0i" (make-rectangular 1 1) "1+1i" "1+i")
 ;;(test-numeric-syntax "#i1.0+1.0i" (make-rectangular 1.0 1.0) "1.0+1.0i" "1.+1.i")
 
+(define-syntax test-precision
+  (syntax-rules ()
+    ((test-round-trip str alt ...)
+     (let* ((n (string->number str))
+            (str2 (number->string n))
+            (accepted (list str alt ...))
+            (ls (member str2 accepted)))
+       (test-assert (string-append "(member? " str2 " "
+                                   (let ((out (open-output-string)))
+                                     (write accepted out)
+                                     (get-output-string out))
+                                   ")")
+         (pair? ls))
+       (when (pair? ls)
+         (test-assert (string-append "(eqv?: " str " " str2 ")")
+           (eqv? n (string->number (car ls)))))))))
+
+;; these all assume double precision, need to add alternatives for single
+(test-precision "-1.7976931348623157e+308")
+(test-precision "4.940656458412465e-324")
+(test-precision "9.881312916824931e-324")
+(test-precision "1.48219693752374e-323")
+(test-precision "1.976262583364986e-323")
+(test-precision "2.470328229206233e-323")
+(test-precision "2.420921664622108e-322")
+(test-precision "2.420921664622108e-320")
+(test-precision "1.4489974452386991")
+(test-precision "0.14285714285714282")
+(test-precision "1.7976931348623157e+308")
+
 (test-end)
 
 (test-end)